Abstract: The calculation of internal force's envelope diagram fora bridge under moving loads is a typical problem which can besolved by the computer simulation. In this paper, a computersimulation is made forthe vehicle load moving on a highway's bridge with a small steplength. For each step, the internal force diagram is plotted. When the vehicle load moves through the bridge,these internal force diagrams will fill in a region. The up-lowerboundary of the region is just the internal force envelope diagram. Thealgorithm in this paper does not depend uponthe concept of the influence line, hence the calculation is simple.