The invention discloses a system availability efficient simulation method based on a reachable matrix and discrete event driven. The available state of a system is converted into the reachable state of the system after failure and maintenance of each unit, and an availability simulation value is obtained through the discrete event driven and many times of simulation, thereby improving the efficiency and effect of the availability simulation of a complex system. The method comprises the following steps: step 1, converting a Reliability Block Diagram of the system into an adjacent matrix for thenormal operation of the system; step 2, defining the number of times of simulation convergences; step 3, carrying out failure and maintenance time sampling on each unit in the system, and sequencingby time and generating a discrete event set; step 4, changing elements associated with the unit in the adjacency matrix based on unit failure events and maintenance events; step 5, solving the reachable matrix of each discrete event occurrence moment, and determining the available state of the system according to a reachability relationship between a start unit and an end unit; and step 6, calculating the availability of the system at each moment according to a result of many times of simulation, and generating an availability simulation curve.