The double-index of temperature control for concrete pouring surface, i.e. the corresponding allowable temperature and temperature gradient at typical age before the pouring surface reaches its maximum temperature, was obtained by maximum entropy method (MEM). The measured temperature of the concrete pouring surface of an extra-high arch dam in high temperature season in southwest China was taken as background in this research. In order to obtain highly accurate maximum entropy probability density function, particle swarm optimization (PSO) was adopted to optimize and solve the Lagrange multiplier coefficients. It is indicated by the engineering practice analysis that sample distribution type needs not to be assumed in advance for MEM, and highly accurate probability distribution density function can be obtained by the calculation of the numerical eigenvalue of each basic random variable. The obtained double-index is feasible.