Considering the randomness or interval character of physical parameters and applied loads of composite pressure vessels ( COPV ) , the COPV reliability‐analyzing model is built . And then the computational expressions for the mean value , standard deviation and deviation of fiber stress are deduced by the random factor method . The probabilistic and interval reliability designs on COPV are implemented by utilizing the probabilistic method and interval method combined with the improved particle swarm optimization ( PSO) algorithm , respectively . The influence of fluctuation of structure uncertain parameters on COPV design thickness is inspected . Finally , examples are given to demonstrate that both probabilistic and interval reliability methods can satisfy the safety requirement and both are of higher rationality than the traditional safety factor method . And probabilistic method has a lower relative error but a higher computational complexity contrasted with the interval method .