Aiming at the problem that it is difficult to precisely control the mold fit performance of the aero-engine combustion chamber cap, the numerical analysis of the viscous pressure forming is carried out. According to the difference of forming pressure and holding time, 7 groups of finite element models of viscous pressure forming are established to explore the influence of forming pressure and holding time on distribution of wall thickness and springback. The results show that under the condition of constant pressure holding time, the wall thickness difference of cap viscous pressure forming decreases from 2% to 0.7% as the increase of forming pressure from 20 MPa to 80 MPa. When the forming pressure is 80 MPa, the springback reaches the lowest value, which is about 47.1% lower than that of 20 MPa. In addition, under the condition of constant forming pressure, the springback reaches the lowest value when the holding time is 0.10 s.  Compared with the holding time of 0.06 s, the springback value decreases by about 60.8%. Within the selected parameters range, increasing the forming pressure can improve the mold fit performance of the cap viscous pressure forming. Compared with the holding time, the forming pressure has a more significant effect on the defect characteristics of the cap. Considering the risks of cracks and wrinkles caused by high thinning rate and thickening rate, the final selection of forming pressure is 80 MPa, and the holding time is 0.10 s.