High magnet-conductive Ni-Fe alloy coil(1J77) was researched, B-H curve of which was defined; by analyzing the operation process under square wave voltage of 1J77, it was proved have fast response. Working principle under the different input situations of DC, low and high frequency of SVCT series fluxgate voltage sensor was introduced by taking SVCT.1000B product as example. The product satisfied actual applying required, and passed normal environment and EMC reliability test.