The quarter-waveplate is one of the most important optical devices in all fiber optical current transducer (AFOCT). Taking into account the temporal coherence of SLD, the detailed analysis and calculation of the entire spreading process of the beam are completed by using Jones Matrix. The expression of optical inten? sity depending on the parameters of the quarter-waveplate can be set up. According to the principle of closed-loop circuit, the law about quarter-waveplate affecting the signal-to-noise ratio (SNR) and scale factor of AFOCT is disclosed and verified by experiments. Optimum design of the quarter-waveplate which is based on the temperature characteristic of the Verdet constant is also shown.