Collecting the pairs of the numerical weather prediction model daily precipitation forecast results and observed data, the uncertainty index U is set， according to the relative position that the joint entropy exists between maximum joint entropy and minimum joint entropy， so as to analyze the uncertainty of NWM precipitation prediction results quantitatively. Preliminary results show that: U is closely linked with the random deviation error of the model forecast, but has nothing to do with the system deviation error; the uncertainty index of the NWM precipitation forecast is over 60%; it has both reference significance and uncertainty; there is bigger uncertainty to forecast the heavy precipitation. As for the continent wide 24 h precipitation forecast in China, the uncertainty of the Japan model is smaller than the T213. When a 40 to 80 mm precipitation appears in the forecast of Japan Regional Spectral Model (RSM), it has the higher reference value than the T213.