Abstract：The present theory formula for calculating the diffusive area of serum mainly covers spherical diffusive formula and cylindrical diffusive formula. In these theory formulas，the change of serum¢s viscosity in the process of injecting sand layer is not considered. It is well known that most of serum¢s viscosities varies with time. The theory formula for calculating the diffusive area of serum is deduced and obtained. The theory formula considers the change of serum¢s viscosity for the process of injecting sand layer. Thus，the calculation errors are removed that exist in present calculation，which are caused by assuming that the viscosity is unchangeable. The results that are achieved from the theory formula match well with that of field test. The conforming sand layer widely exist in nature，and it has good geometrical property. According to the property of conforming sand layer，the theory formula for conforming sand layer is simplified and the methods of achieving the function between serum¢s viscosity and time are presented. The method can avoid the unfavorable matter that the serum¢s viscosity must be measured in each construction. Therefore，the costs for engineering can be considerably saved.
