Structure entropy has become an important index of the stability and adaptability of software systems. This paper is to find out the basic trend and evolution laws of structure entropy with software scale. Experiments were carried out by using a large number of different scale software and a series of different versions. The authors found that software entropy gradually increases with the scale of software, and standard entropy has been remained around 0.8. This conclusion provides a favorable clue for the follow-up research on the measure of software stability and adaptability.