The biasing effect of Zeeman laser gyroscope affected by He-Ne gas was studied theoretically and experimentally. It is found by theoretical analysis that the He-Ne gas considerably affects the biasing value of the gyroscope,which indicates that the value linearly depends on the pressure of the gas with the constant ratio. The biasing value was measured under different pressures,and the comparison between the theoretical result and the measurement shows good agreement,and it verifies the validity of the theoretical model. The research provides basis for choosing the intensity of pressure of He-Ne gas, which is practically important for Zeeman laser gyroscope design consideration.