Earthwork is an important content in engineering construction. This paper explores a scientific method of earthwork calculation, designs and develops corresponding software which accurately calculates the earthwork, effectively reduces engineering investment and budget deviation. It mainly studies the three-dimensional model building by the complex form through two period observations. The model building methodology mainly adopts the elevation of two period observation data`s mutual insertion to establish the two period superposition triangle net. And then getting the method of calculating tri-prism volume formed by triangular. This method, the volume calculation principle is simple, high precision, and the design of the special software system is based on the above method. At present, the software system applies in the engineering, and it has computational fast, convenient, results accuracy and reliable etc., advantages.