In this paper, the hybrid physical model is established based on the equivalent circuit for describing dynamic radio-frequency (RF) field buildup and the particle-in-cell (PIC) method for describing two-sided multipactor discharge in plate cavity. By using our built 1D3V-PIC code for multipactor discharge and fully equivalent circuit code for RF field buildup, the influence of multipactor discharge on the dynamic process of RF field buildup is numerically investigated and analyzed in detail under the condition of cavity with different Q-values. The numerical results could be concluded as follows. Under the condition of no multipactor discharge in dynamic process of RF field buildup, the higher the Q-value, the longer the buildup-time is. The input energy is equal to the sum of stored energy and consumed energy in cavity, the speed of energy storing is higher than the speed of energy consuming at the beginning stage of RF field buildup and then the speed of energy storing becomes lower than the speed of energy consuming. When the process of RF field buildup is finished, the average power of input is equal to the average power of consumed power in cavity. Under the condition of multipactor discharge loading in dynamic process of RF field buildup, the higher the Q-value, the later the start-time is and the longer the interaction time-interval of multipactor discharge is. The bigger the area of secondary electron emission, the higher the peak-value of secondary electron current is. The failure of RF field-buildup is caused by the continuous loading of multipactor discharge. The higher the Q-value or the bigger the area of secondary electron emission, the lower the probability of RF field buildup success is. The simulated results could partly provide a reference for engineering design.