The accurate perception of distribution system depends on the state estimation and the successful operation of state estimation depends on the measurement system. This paper proposes a novel method concerning fast decoupled identification of critical measurement. The method can identify all critical measurements and critical measurement sets in a decoupled and non-iterative manner. On one hand, only state estimation calculation relevant to active power measurements is needed, which greatly reduces the computational scale. On the other hand, no iteration is needed and relevant strict theoretical proof is given. Besides, a three-phase m-1 robust measurement placement model is proposed, aiming to guarantee the network observability in the case of any measurement being lost. Test results based on the modified IEEE 37 node test feeder show that the decoupled method attains much faster speed than coupled one and the robust measurement optimal placement significantly reduces the risk of unobservability.