From the long-term continuous automatic observation to the global solar radiation Q and PAR Q_PAR in Beijing, Yantai, Zhengzhou and Jinan during the growth period, it is discovered that the value of Q_PAR/Q is about stable. The effect of the season and cloud amount to it is nominal. The variation at the lower solar elevation is caused by the cosine response of the instrument. The current instrument manufacturing technique can not satisfy the need of the study on effect of the solar elevation on Q_PAR/Q. The analysis of the observation data gives that some formula parameters in abroad is higher,the computation formula Q_PAR = 0.42 Q which suitable to the east plain in China is obtained.