A Dual-Wavelength Laser (DWL) based on the Nd∶YVO4/Nd∶GdVO4 combined crystal was presented. The thermal effect and the frequency-separation tuning mechnism of the DWL were investigated. In the experiments, with fixed pumping power, when the heat sink temperature of the combined crystal increased from 5 ℃ to 40 ℃, the frequency separation of the DWL signal decreased from 351.1 GHz to 316.1 GHz, the frequency separation decreasing rate was measured as -0.95 GHz/℃. For the experimental results, it was found that the wavelength shifting of the DWL signal is mainly caused by the Emission Cross Section (ECS) spectra shifting of the laser crystals with the temperature varying, and the change of the frequency separation is caused by different ECS spectral shifting rates of the crystals. The analysis results are in good agreement with the experimental results.