The invention discloses a space flexible mechanism dynamic reliability analysis method based on PSO. The space flexible mechanism dynamic reliability analysis method includes the specific steps of (1) building a time-variation reliability model of an in-orbit space flexible mechanism according to the reliability modeling theory, (2) carrying out time discretization on the time-variation reliability model, and dividing the time-variation reliability model into a plurality of time points t1, t2, ..., tn, (3) building performance functions corresponding to the time points, and then building an association coefficient rou(t, t+deltat) between t and t+deltat by deriving normal vectors of the performance functions, (4) obtaining reliability coefficients beta1, beta2, ..., betan with the PSO, (5) calculating the up-crossing rate in cooperation with the association coefficient rou(t, t+deltat) between t and t+deltat, and (6) calculating the failure probability with the up-crossing rate method. By means of the space flexible mechanism dynamic reliability analysis method, the accuracy of a high-dimensional nonlinear flexible time-variation reliability analysis result is improved.