On the condition of radar net, the main factors that effect the precision of guidance radar data fusion is discussed in detail, and several main error sources are analyzed. When one radar station can be supported by a few other radar stations in the guidance radar net, by adopting some high precision approaches such as ellipse globe and linear bias, the data fusion process formula and error matrix of this radar station are derived. The Monte Carlo simulation approach is used to analyze the precision of data fusion in guidance radar net, and the result analysis of simulation is also given.