In the all-fiber optical parametric oscillator (OPO) based on four wave mixing (FWM), the signal laser gets effective gain only in a short distance. The increasing trend of signal laser in photonic crystal fiber (PCF) is measured by optical time domain reflecting (OTDR) technology, and the backscatter signal light is measured. The parametric gain distribution in PCF is calculated, and the distance between the start of the PCF and the sharp drop in the parametric gain is defined as the effective interaction length. Moreover, the results are verified by PCF heating experiment. The results show that there is only 10 m of the effective interaction length in the 34.5 m PCF. The signal laser keeps growing exponentially only in the first 10 m PCF, and cannot increase any more in the rest 24.5 m PCF for the signal and the pump light is no longer meet the phase matching conditions. An effective method for measuring the parametric gain distribution is proposed, and the result is useful for optimizing the configuration and improving the efficiency of all-fiber OPO.