Stess spectrum serves as prerequisite of analysis on fatigue failure, which is the major cause of malignant industrial accident. The dynamic load coefficient is calculated by allowing for dynamic response of steel structure in complete working cycle and regard steel structure and load as a vibraton system of two degrees of freedom. The load date and its statistical characteristic is collected.through observation of 600t-176m gantry crane in some ship-building base. Then based on Monte Carlo method, the load is simulated and loaded on finite element model. Finally the stress spectrum of key parts of structure is calculated.