A pyranometric method for determination of sunshine duration is proposed. Global solar radiation measured with pyranometer and direct solar radiation measured with pyrheliometer at Solar Energy Resource Observation Station in Lijiang, Yunnan province is used. The clearness index is set as an independent variable which is calculated from the ratio of global solar radiation to extraterrestrial solar radiation, the sum of the time, for which the direct solar irradiance exceeds 120 W/m2, is set as the sunshine duration reference. The relationship between clearness index and sunshine duration is analyzed, and the piecewise function is obtained by segmentation fitting for estimating sunshine duration. Though the verification of the observation date in Lijiang, the sunshine duration reference and sunshine duration estimated by the method have good correlation, the coefficient between the two sunshine durations is up to 0.992 3, the difference of the month summation is 0.07 h, the root mean squared deviation of daily differences in a month is 0.5 h. And the method has a good performance to estimate the sunshine duration.