Optical polarimeter module combined with its electrical drive circuit forms a polarization detector. The states of polarization (SOP) (S0, S1, S2, S3) can be obtained by a DSP treatment through the 4 output voltages (V1, V2, V3, V4) from the polarization detector. A calibration matrix should be established before the polarization detector can work well. We design a calibration method named vector projection (VP) which is based on the theorem of vector projection, by which calibration matrix can be calculated for the polarization detector module. The experiment showed that using our method the SOP accuracy is ±2%, and DOP accuracy is 3%.