Using the surface observation data, radar data, FY2G satellite cloud images and ECMWF_HR data, the extra torrential rain of Typhoon Lekima in Shandong in 2019 is analyzed. The results show that during the landfalling and northward moving process of Lekima, the cold air invaded into the center from west, southwest and south of the typhoon, which gradually weakened its warm center structure and during the extratropical transition period from 20:00 of 10 August to 08:00 of 11 August. The extra torrential rain in Shandong occurred during the period of 12 hours ahead and 6 hours later of the extratropical transition. Before the transition, the precipitation in Shandong was mainly caused by the interaction between the spiral cloud belt of the typhoon and the tail clouds of the low trough, but the precipitation during the transition was caused by the cold air intrusion which made the clouds outside of the typhoon develop into mesoscale convective convergence. Before the transition, the wind speed of 800 to 500 hPa in the troposphere was small, and that of 500 to 250 hPa was large. There were warm advection and ascending motion in the air layer, and the main precipitation came from convective precipitation in the warm zone. After the transition, the wind speed of 800 to 500 hPa was large, and that of 500 to 250 hPa was small. The ascending motion lain from 500 hPa to the ground, and the descending motion was above 500 hPa. The precipitation belonged mainly the convective precipitation near the baroclinefront area. When the cold advection appeared between 500 hPa to the ground, the wet layer became thinner and the precipitation tended to weaken. The heavy rain appeared in the energy front on the northwest direction of the typhoon center, which was consistent with the divergence convergence of water vapor flux at 850 hPa.