Under-voltage and under-frequency load shedding are two common emergency control approaches to ensure the voltage stability and the frequency stability in power systems. Because the under-voltage and under-frequency phenomena are often coupled with each other in practice， single scheme for only one situation is not capable of resolving both frequency and voltage problems at the same time. Therefore， a coordinated load shedding scheme in which both situations are considered is proposed. Adopting the idea of‘searching and verifying'， this scheme formulates load shedding as a satisfiability checking problem. First， based on their own complexity， the constraints which are determined by voltage stability and frequency stability requirements are divided into two parts and assigned to either‘searching' or‘verifying' stage respectively. The ones which can be easily depicted by a set of Boolean expressions should be put into the‘searching' stage， while the others should be put into the‘verifying' stage. Second， OBDD algorithm is applied in searching， and the solutions that do not satisfy the constraints in the‘searching' stage are filtered out， therefore， strategy space is reduced. Third， any strategy from the reduced strategy space is further checked by the remaining constraints in the verifying stage in order to get a satisfying control strategy. In the last part of the paper， some simulation results are exhibited to demonstrate the feasibility of the scheme.