Power transfer limit (PTL) calculation plays an important role in assessing power network capability under certain constraints of system security and stability. However, the impact of ambient factors, which is different due to variations in time and space, is ignored in traditional methods to obtain PTL, thus inducing errors. Furthermore, system operation based on traditional PTL results may increase system security risks, particularly in the case of power flow congestion under heavy loads. Therefore, this paper proposes a decentralized PTL calculation method with improved optimal power flow model, which allows for the effect of the ambient factors characterized by the balance of heat absorption and dissipation for overhead conductors. The ambient factors of overhead transmission lines and the temperature of overhead conductors are involved as independent variables and state variables, respectively. Moreover, the sequential optimization problem is decomposed into several subproblems by the optimal conditional decomposition to deal with the temporal coupling constraints, and a parallel decomposition framework is used to solve multiple subproblems in parallel. Finally, the proposed method is implemented on two test systems under varying ambient factors, demonstrating the efficiency of the proposed method and the significant impacts of spatial and seasonal differences on PTL results.