The effects of light source spectrum on performance of the optic fiber voltage sensor（OFVS） are analyzed theoretically. Especially， the coherent multi-path effects， caused by the back reflection and the backscatter generated by the interface of the polarizer， the quarter wave plate， the electro-optic crystal and the polarization beam splitter are analyzed with Jones matrix method. Then the effect of the secondary peak of the light source coherence function on the performance of OFVS is also investigated. According to the above theoretical analysis， the spectrum requirements of OFVS for measurement are given. A test is conducted with a light source meeting above spectrum requirements， and the results show that the performance of OFVS can meet application requirements.This work is supported by State Grid Corporation of China.