By using of the helicity theory,humidity and stability condition,the torrential rain in the southeast and the northern of the central Shandong province during August 11—12,1999 was diagnosed.The analysis results show that the rainstorm is produced in higher temperature,higher humidity and unstable atmosphere.There is higher positive k helicity at 500hPa and at low levels.Rainstorm occurs near the positive center of helicity at 850hPa.The variation of helicity indicates weather systems moving developing and falling area and intensity of torrential rain.Helicity also reflects the distribution of vertical moving and rotation status of atmosphere.