Abstract: A test sequencing problem of multi-mode system is studied.Based on the dependencies between failure sources and available tests at different mode，combined with the prior probability of failure sources，the cost of tests and of mode changing，a mathematic model on this problem is constructed.To extract an optimal test sequence which ensures to accomplish the system fault detection and isolation quickly under the least average testing cost，a quasi multi-step look-ahead search algorithm is presented.This algorithm is based on the existed one-step look-ahead search algorithm and multi-step look-ahead search algorithm.It employs the information gain as its heuristics and achieves an efficient tradeoff between optimality and computational complexity.A real test case is given to verify the presented approach.The experimental results show that the presented algorithm can efficiently solve practical problems.