The method of measuring the electric field of a tapered fiber-slab coupler(TFSC) is studied. The influence of the parameters of TFSC on the propagation performance is discussed in detail in order to design a TFSC sensor with better parameters. Firstly, the simulation platform is simulated in RsoftBPM, and the influence of the coupling parameters on measurement accuracy, measurement range and size of the equipment is analyzed qualitatively according to the coupled-mode theory. Then, the design principle for the sensor of improving measurement accuracy or increasing measurement range is proposed and a set of design examples is given. Finally, the distortion of the original electric field is analyzed, and the coupler structure is improved by simulation.