Leading time as a variable of fault detection rate into the G-O Model of Software Reliability Growth Model(SRGM), we build a new Software Cost Model. This Software Cost Model considers the remaining fault detection probability of software system users after the software’s publishment. It is a model that considers error is not completely found, so the model is more practical. The article is based on the least costs of the software as the best condition and talks about the relationship of publishing time and costs. At the end of this article, examples are showed to explain the publishing time changing by parameters.