The expression of formal error of deflection of the vertical (DOV) surveyed by classical global navigation satellite system (GNSS) is derived. A small scale control network parameters transformation method for calculating DOV of co-located stations is proposed. Based on the observation data of the control network of Nanshan multi-technology co-located stations in Urumqi, the algorithm verification is carried out, and the DOV of several places in the station is surveyed. The results show that the high-precision DOV of the small scale control network is only determined by the three points with the highest surveying precision and the widest coverage area of the triangle. A few points with poor position precision will bring great uncertainty to the DOV value. The consistency between the DOV calculated by small scale control network parameters transformation method and the surveyed result is -2.3±'4.3' (north-south component) and 0.2'±4.6' (west-east component), respectively. Small scale control network transformation method is similar to the classical GNSS method for DOV in the aspect of precision, but the former is more convenient. Considering that local surveying are carried out in irregular intervals in those multi-technology co-located stations, a zero-cost and long-term DOV monitoring in multi-technology co-located stations can be realized by the method.