Matsa (0509) is an extratropical transition (ET) typhoon which made landfall in Liaodong Peninsula in 2005. The PSU/NCAR non-hydrostatic mesoscale model MM5 is used to simulate the heavy rainfall process in Shandong Peninsula and Liaodong Peninsula during 8－9 August 2005. Several sets of numerical simulations are performed to study the impact of cold air on the rainfall of landfall tropical cyclone (TC) over Liaodong Peninsula. Results indicate that the intensity of convective cloud clusters, vertical upward motion and precipitation in Liaodong Peninsula are increased obviously when the cold air is invading the external area of TC. Both of the weak cold air in the lower and middle troposphere and the rather strong cold air in the upper troposphere can lead to notable intensification of radar reflectivity and ascending motion in Liaodong Peninsula, which is favorable for enhancement of precipitation there. The latter effect is more significant than the former one. Without the impact of cold air in the lower and middle troposphere, the heavy rainfall center would occur in the north of Liaodong Peninsula. The severe cold air in the lower and middle troposphere can result in the weakening of vertical velocity and TC circulation over Liaodong Peninsula, thus TC rainfall decreases remarkably in that area.