Abstract: To solve the safety assessment problem of large-scale in-service mechanical equipments’ steel structure , the probabilistic safety assessment method was proposed under the dead load and fatigue conditions with the aid of second level method and Monte Carlo method. Amended equation of it was established according to the three-period corrosion law in consider of corrosion lost under marine environment with which the ship loader bridge and wagon tipper in Qingdao Port were calculated and analyzed. Both definite safety assessment and the probabilistic safety assessment results were obtained. The results from the two assessment methods were generally closed but still there was little difference between them. The primary relation-approximately logarithm relationship,between the definite safety assessment and the probabilistic safety assessment was obtained from the comparative investment between the results of two kinds of assessment methods. The relationship obtained is the key point for the effectively application of the concerned probabilistic safety assessment method to in-service mechanical equipments’ steel structure.