Aiming at the abnormal phenomenon of negative statistical line loss occurring in some interprovincial ultrahigh voltage (UHV) transmission lines under light load, the influence of ambient temperature on measurement of UHV transmission line is analyzed in this paper. Firstly, the UHV transmission line of NanyangJingmen is taken as an example to establish a mathematical model of the influence of temperature on equivalent dividing capacitance of capacitor voltage transformer (CVT). Then, the relationship between the ambient temperature and the equivalent dividing capacitance of CVT is deduced. In the end, based on the historical temperature data, the influence of ambient temperature on the voltage division ratio of CVT is calculated. It is shown that the error of CVT voltage ratio caused by the environmental temperature may result in the negative statistical line loss of UHV transmission line when considering the discrepancies of CVT parameters.