Mergers of cells in a severe convective weather on 22 July 2008 are simulated and analyzed by Mesoscale Model 5 (referred to as MM5）and radar network data. Observation results show that, the horizontal scale of the echo above 30 dBZ, which represent the small cells, is about 10 km, and the small cells that the echo centers are 20 km apart merge into a larger cell at dozens of km of horizontal scale. Mergers begin from the peripheral radar echo, and then strong central radar echo merges at the low level, at last, the acreage of strong radar echo increases after the merger. The contrast between the observations and the simulation results shows that they are consistent. Analysis on the simulation results of two kinds of cell mergers at different development stages based on the third network model output shows that, while the cell pairs are with almost the same intensity, cells would develop after merger; while one of the cell pairs is in stronger development however the other one weaker, the stronger cell would keep on development and the weaker cell would die out. During the merger, a new cloud water center appears in the low convergence region between the cell pairs, and would replace the two cloud water centers of the former cells, or the new cloud water center would merger with one of the old cloud water centers while the other old cloud water center disappears. The analysis of the simulation results also shows that, the cell merger would lead to the cloud top lifting and the increase in the radar echo, content of cloud water and ice, surface rainfall.