In recent years, dual-wavelength lasers are used in a wide variety of applications in interferometric measurement, optical communications, microwave and THz wave generations, and optical frequency combs. A variety of methods to realize dual-wavelength lasers have been proposed, including fiber lasers, Y-branch integrated lasers, and two-section Distributed Feedback (DFB) or Distributed Bragg Reflection (DBR) lasers. In addition, the whispering gallery mode microcavity laser has great application potential in photonic integration due to its small mode volume, high-quality factor, and simple manufacturing process.In this paper, we design and fabricate a square microcavity laser with a current injection window in the center and four corners to achieve tunable dual-wavelength lasing. By simply changing the injection current, we realize dual-mode lasing with a wavelength interval tuned from 0.18 nm to 0.1 nm, and an intensity ratio less than 4 dB.A two-dimensional finite-element method is used to simulate the TE modes of the square microcavity. It can be seen that along the lines connecting the midpoints of the adjacent sides, the fundamental mode and the first-order mode show the strong field and the weak field distribution, respectively. Due to the incomplete overlap of the mode field distributions, there is less mode competition between the fundamental mode and the first-order mode, and the quality factors are 6.994×10⁴ and 1.838×10⁴, respectively. Taking into account the vertical radiation, material absorption, and manufacturing process losses, the two modes have similar quality factors to achieve dual-mode lasing.Based on the mode field distribution, a deformed square microcavity laser is designed with a current injection window in the center and four corners, which induces a refractive index step. For a square microcavity with a side length of 30 μm, the numerical result shows that the mode wavelength interval can be reduced from 1.07 nm to 0.11 nm when the refractive index step increases from -0.005 to 0.003.Next, a dual-mode square microcavity laser with a current injection window in the center and four corners is successfully fabricated with a side length of 30 μm, and a waveguide of 3 μm. The maximum output power coupled into a multimode fiber is 1.04 mW when the injection current is 77 mA. The series resistance is 11.4 Ω, and the threshold current is about 9 mA. When the injection current is increased from 41 mA to 53 mA, the wavelength interval of the microcavity laser decreases from 0.18 nm to 0.1 nm. Meanwhile, the intensity ratio is less than 4 dB. Since the lasing wavelength of InP-based lasers changes with temperature at a rate of 0.1 nm/K, the temperature difference between the injection window and the non-injection window can be estimated to be 2.5 K when the injection current is 50 mA. Compared with the simulation results, the refractive index step corresponding to the dual-mode interval of the microcavity laser increases from 1×10^-3 to 3×10^-3 with the increase of the injection current. It indicates that the refractive index of the microcavity laser is mainly affected by the temperature distribution. According to the experimental and simulation results, the refractive index step and the current have a quadratic relationship. In addition, a period-one oscillation phenomenon appears due to the further reduction of the dual-mode interval.For comparison, a square microcavity laser with a square-ring-patterned contact window is also fabricated with a side length of 26 μm, and a waveguide of 2.5 μm. The dual-mode interval gradually increases with the increase of the injection current. When the injection current is increased from 62 mA to 85 mA, the dual-mode interval can be tuned from 0.202 nm to 0.284 nm. It further verifies the conclusion that the refractive index is mainly affected by the temperature distribution.In conclusion, a square microcavity laser with a non-uniform injection window in the center and four corners is designed to realize dual-mode lasing with tunable intervals. When the injection current is increased from 42 mA to 53 mA, the wavelength interval decreases from 0.18 nm to 0.1 nm. The proposed square microcavity laser with a current injection window in the center and four corners provides a light source with a tunable interval for the generation of microwaves, optical frequency combs, and the potential chaotic lasers.