Based on conventional observation data and 1°×1°FNL global reanalysis data from NECP/NCAR, the wet Qvector method is applied to diagnose a persistent torrential rainfall event in the northwest of Shandong Province in 2012. The results are as follows. The torrential rainfall event appears under favorable circulations and the precipitation area is concentrated and has obvious mesoscale features. The wet Q vector method serves as a powerful tool to analyze heavy precipitation area. The wet Q vector divergence field is correlated well to heavy precipitation area from 925 hPa to 850 hPa, while the torrential rain does not always appear in the negative center of wet Q vector divergence. It sometimes appears in the negative area of wet Qvector divergence with high gradient. The superposition area of the the positive center of wet Qvector vorticity and the negative center of divergence at 700 hPa is advantageous for developing mesoscale system of low value, which corresponds well to the torrential rainfall area in the next 6 to 12 hours. The function of frontogenesis difference using the wet Q vector is superior to the function of frontogenesis in forecasting where the torrential rain will take place.