Based on the theoretical process model for positive photoresist, ray-tracing algorithm is used to calculate the time evolution of surface profiles produced by a locally dependent surface etching phenomenon. Depending on the simulation programs established, the final developed surface profile can be computed from any given distribution of exposure energy and development time. This provides a convenient way for investigating laser direct writing process. By introducing a threshold approximation in development process, the relationship between the initial distribution of exposure energy on the upper surface of the resist film and the surface profile after development is deduced, which provides an effective theoretical approach for laser direct writing technology.