The variation of photosynthetically active radiation (Q_p) and ratio of Q_p to broadband solar radiation (R_s) at Sanjiang in Northeast China were analyzed on the basis of in situ data measured during 2005-2011. The results indicate that the seasonal and diurnal variation of Q_p are similar to those of R_s. The daily values of Q_p range from 0.11 to 60.47 mol m^-2, with an average annual value of 23.76 mol m^-2. The daily values of Q_p/R_s range from 1.52-2.07 mol MJ^-1, with an average annual value of 1.91 mol MJ^-1. The seasonal variation of Q_p/R_s is similar to that of Q_p. Q_p and Q_p/R_s reach a maximum in summer and a minimum in winter. An estimation equation for Q_p was established on the basis of the optical air mass, clearness index, and Q_p data measured in 2011. Evaluation of the results shows that this estimation equation can be used to calculate historical Q_p data with a relative error within 5.7%.