Space-time coding has the advantage of simplicity on coding, easy to achieve full diversity transmission, and low complexity on decoding. Based on the estimated inequality of the expectation of a random variable function, block error rate of the orthogonal space time block decoding is analyzed, and a new upper bound on the symbol error rate is achieved. Precoding criterion on orthogonal space time coding has been proposed based on the new upper bound. The solving complexity of the precoding matrix is reduced effectively. Simulation results show that bit error rate of the precoded system is lower than that of the without precoding.