The failure of an equipment is usually a random process, so the preventive maintenance (PM) interval of the equipment is mainly formulated by maintenance engineers according to their experiences, or by the specification of PM interval given by the manufacturing enterprise, which results into two main problems of over maintenance and insufficient maintenance. In order to optimise the preventive maintenance interval, a time delay model, in the condition of Poisson process defects and perfect inspection was established based on the maintenance data of recorded failures and inspection data. Then the failure process in case of perfect inspection was simulated by computer programs. The simulation results prove the validity of the established time delay model. The established time delay model and the simulation system demonstrate intuitively the failure process, reveal clearly the failure mechanism, and solve the difficulty of model validation in case of no failure data record. The failure process model is benefitial to make maintenance decision.