A measurement model of gas volume fraction（GVF） was deduced based on the energy equation，and the relationship of friction pressure drop between two vertical sections of U tube was considered.The measurement errors were investigated through experiment under low pressure annular-mist and wispy annular two-phase flow.The effects of gas mass friction，gas volume friction and flux density on gas volume fraction were analyzed and gas volume fraction was corrected with flux density.This correction improved the accuracy of gas volume fraction，and the correction method was verified through another independent test.