Calculation of the forest land parcel surface area is vital to the confirmation, segmentation and assignment of forest ownership. Therefore, the method calculating irregular forest land parcel surface area, which is based on the principle of space triangle area calculation, establishes automatic extraction of irregular forest parcel boundary and structures Triangulated Irregular Network (TIN) model by interpolating feature points based on Thin Plate Spline (TPS) function. This paper expounds the feasibility of this method and complete calculation process. By experimental comparison, this method can reduce about four-fifths of field data collection effort in the case of no accuracy loss in calculating results. Simultaneously, this method extracts boundary automatically without manual drawing boundary lined, improves degree of automation in computing within the industry greatly.