In grinding and polishing of big mirrors using active stressed lap, lap surface accuracy restricts mirror accuracy and processing efficiency chiefly. Lap surface error, which is the mismatch between the mirror and the lap, causes partial pressure concentration and poor efficiency in grinding or polishing. Different from surface representation of active stressed lap using PV and RMS, structure function could describe average variance over all regions on lap in a given separation. This paper illustrates an efficient technique for representing active stressed lap surface error using structure function and relating bending model to the representation. Example of this procedure is described and results show that this method could represent lap surface exactly; meanwhile the surface accuracy after compensating residual error caused by lap sag could achieve allowable error specification for big mirror manufacturing.