The response characteristic of near infrared detector PDA400 was measured, then the theoretical analysis and experimental study,which focused on the relation between detector's response characteristic and metrical result of continuous-wave cavity ring down technique, were both carried on. With the linear response function, the output signal of PDA400 was deducted when ring down signal was used as input.In this case,analusis shows that the signal shape of PDA400 has turned from single to double exponential function. After the response times of PDA400 in different gains were measured, the fitting method of ring down signal in two typical detector's gain(20 dB and 40 dB) was studied, and two kinds of methods were proposed and tested with experiments. Furthermore, when considering detector's nonlinear response, the method of data processing is discussed too.