In this paper, we redefine and calculate boreal summer intraseasonal oscillation(BSISO)index by making empirical orthogonal decomposition (EOF) using the OLR data and the 850 hPa zonal wind velocity u data over 10°S-50°N, 40-160°E from May to October in 1981-2010. The evolution characteristics of BSISO and its influence on summer precipitation in North China are analyzed. The results are as follows: (1) there are two obvious low frequency signals in the Indian Ocean-Northwest Pacific region in summer. One is BSISO1, which is inclined from northwest to southeast and propagates from the tropical Indian Ocean to northeast with an oscillation period of about 45 days. The other is BSISO2, which is inclined from southwest to northeast and propagates from the Northwest Pacific to the northwest with an oscillation period of about 15 days. (2) BSISO affects the summer precipitation process in North China mainly by influencing circulation anomalies and water vapor transport anomalies. At 500hPa, BSISO signal will cause the North-South movement and intensity change of the subtropical high in the east of North China to affect the summer precipitation process in North China. At 850 hPa, BSISO signal will affect the water vapor transport to North China and the convergence and divergence through the associated cyclonic or anticyclonic anomalous circulation to influence the summer precipitation process in North China. (3) Although MJO signal exists throughout the year, its amplitude is the largest in winter half year, especially in winter, and the minimum in summer. The amplitude of BSISO signal is the largest in summer half year, especially in summer. Therefore, using the low frequency signal of tropical atmosphere to predict the extension period, winter half year can focus on consideration the impact of MJO and summer half year focus on consideration the impact of BSISO.