The　mode　splitting　induced　by　electro-optic　birefringence　in　an　P-I-N　InGaAs/GaAs/AlGaAs　vertical-cavity　surface-emitting　laser　(VCSEL)　has　been　studied　by　photocurrent　difference　spectroscopy　(PCDS)　at　room　temperature.　The　mode　splitting,　anisotropic　broadening　width,　and　the　anisotropic　integrated　area　of　the　two　orthogonal　polarized　modes　for　a　VCSEL　device　are　determined.　The　mode　splitting　changes　linearly　with　the　injected　current,　which　agree　very　well　with　theoretical　calculations　using　a　Jones　matrix　approach.　It　is　demonstrated　that　the　PCDS　is　a　powerful　tool　to　study　the　cavity　anisotropy　of　a　VCSEL　device.