Based on the calibration method of rainfall sensors, an evaluating method of measurement uncertainty for errors of SL3 rainfall sensors is described. A measurement model is established, and the error sources of measurement uncertainty are analyzed based on the propagation theorem of measurement uncertainty. According to different evaluation methods, the components of measurement uncertainty are evaluated rationally. An evaluation example of measurement uncertainty for the errors of SL3 rainfall sensors is given in combination with practice. The factors affecting measurement uncertainty are analyzed. The results show that the expanded uncertainty of the errors for rainfall sensors is 01 mm (〖WTBX〗k〖WTBZ〗=2). The standard uncertainty introducing from repeatability of rainfall sensors is the main factor affecting the measurement uncertainty, and the resolution of the rainfall sensors and the counting devices are the next importance factors. The standard uncertainty introducing from the simulation accuracy of automatic rainfall calibrators is the minimal factor.