In allusion to the number of measurements more than that of harmonic sources in the existing optimal placement methods, a method based on independent component analysis (ICA) and priori knowledge is proposed. Suspicious buses can be selected through network topology and the types of loads. The harmonic emission levels of selected buses are calculated by the product of the nonlinear degree and the capacities of loads. The number of buses greater than a preset threshold is obtained. ICA only need the dimension of measurements just equals to that of the harmonic sources, and the measuring device can provide both the harmonic voltage and current, as a result, the number of devices in demand is obtained. The importance degree of each suspicious bus is calculated by the weighted sum of the harmonic emission level and voltage. The buses with higher importance degree are installed measuring devices. The efficiency and practicality of the proposed method has been proved by the cases of the IEEE 14-bus distribution network and a local network.