A single mode-multimode-single mode fiber optic structure and magnetic fluid based all-fiber optic current sensor, which can be interrogated by monitoring the dip wavelength shift or transmission loss of the transmission spectrum, is designed. For the first interrogation method, the current sensitivity of 1243.7 pm/A [77.9 pm/Oe, 1 Oe=1/(4π)×103 A/m] is obtained. For the second way, the sensing property depends on wavelength. By analyzing the rule of the change of transmission loss under different wavelengths, it is found that when a specified wavelength is used, the sensing property is influenced by different wavelengths and the variation of transmission loss is little, leading to a small sensitivity. When the all transmission loss during broad wavelength range of source is employed, the change of transmission loss is obvious, and hence the current sensitivity is greatly enhanced. The obtained sensitivity of 506.2 dB/A (31.7 dB/Oe) is 337 times and 723 times larger than those under 1529 nm and 1611 nm, respectively. Furthermore, the sensitivity of 26.8 V/A (1.7 V/Oe) is achieved by detecting the output intensity using detector with amplified spontaneous emission broadband light source inputting.