The invention provides a Markov state space diagram method for evaluating the reliability of a converter transformer subsystem, which comprises the following steps: analyzing the structure, the operation characteristics, the state and the state interline transfer rate of the converter transformer subsystem, and setting up a Markov state space diagram of the converter transformer subsystem; utilizing a computer to solve the Markov state space diagram through a program; and obtaining and outputting the reliability index of the converter transformer subsystem. The method can fully analyze the structure and the operation characteristics of the converter transformer subsystem, which comprise two fault modes of a converter transformer, the wiring mode of the converter transformer subsystem, a component group operation stopping model and a converter transformer station single-side whole standby mode as well as the fault, the repair and the random transfer process of standby switching installation of the converter transformer subsystem; and by calculating the expected transfer number of borders traversing and surrounding the boundaries of merger states, merger state frequency indexes can be obtained, and the influence of the transfer frequency among the merger states to the frequency indexes can be eliminated, so that remarkable superiority exists during calculating the frequency indexes, and the calculating result is more accurate. The method is suitable for evaluating the reliability of the converter transformer subsystem in a high-voltage direct-current transmission system of single-twelve-pulse and double-twelve-pulse wiring.