Phasor Measurement Unit (PMU) is more and more concerned in power distribution network due to its great benefit. In near future, it may become an essential device in the modern distribution systems. The Optimal PMU Placement (OPP) problem on transmission network for full system observability, was fully done. However, in distribution network the change of configuration in normal operation and contingency modes has a very large impact to find the location of PMUs on network. Some methods have been proposed to solve this problem however the researchers had to spend lot of effort to calculate the system data in all configurations of network. This paper proposes a new solution to find the optimal location of PMUs, which can ensure the full system observability in all operation configurations of distribution network with calculating only based on one configuration. To justify the methodology of solution, the simulations on IEEE RBTS-2 bus and 33 buses distribution systems are presented for experiment. The results of this study show that the proposed solution is effective and feasible.