The analog signal isolator with linear optocoupler is studied theoretically and experimentally, and a linear addition and subtraction compensation algorithm around a critical point is proposed. The linear photoelectric coupler works at the low power consumption end. The final error is an upward or downward parabolic shape. Taking the vertex of the error parabola as the critical point, the linearity of signal transmission can be improved by simultaneously operating the linear addition (or subtraction) of left and right branches. The results show that the linearity is less than 0.003%, extreme accuracy is less than 0.01%. The linearity and accuracy of the analog signal isolator are increased by more than one order of magnitude, which meets the higher requirements of industrial and agricultural production, especially the military applications.