Using a two-dimensional spectral resolving cloud model, the effects of number concentration and size distribution of aerosol particles and relative humidity on mixed-phase convective clouds are investigated, and the changes in aerosol's effects with relative humidity are discussed. The results show that, under the same initial thermodynamic and dynamic conditions, the clean maritime clouds produce raindrops, ice crystals, and graupel particles more effectively in both development and mature stages, resulting in a stronger radar reflectivity. With the increasing in aerosol number concentration, such as in the polluted continental clouds simulated here, excessive aerosol particles limit the growth of cloud droplets and the formation of precipitation particles. It is also found that the aerosol effect on cloud properties and precipitation is strongly dependent on the environmental relative humidity, that is, as relative humidity increases from 50% to 70% at the surface, the cloud changes from shallow cumulus cell to a deep convective cloud. The aerosol effect is not as important in the dry air as in the humid air. This is consistent with the previous studies. The increase of relative humidity makes ice particles form earlier and grow faster, and the cloud anvil expands to a larger area. However, the rain initiating time is more sensitive to relative humidity than to aerosol particle concentration.