Krylov complexity characterizes the operator growth in quantum many-body systems or in quantum field theories. The existing literatures have studied the Krylov complexity in the low temperature limit in quantum field theory. In this paper, we extend and systematically study the Krylov complexity and Krylov entropy in a scalar field theory with general temperatures. To this end, we propose a new method to calculate the Wightman power spectrum which allows us to compute the Lanczos coefficients and then to study the Krylov complexity (entropy) extensively. Compared to the previous studies in the low temperature limit, we find that the Lanczos coefficients and Krylov complexity (entropy) in the high temperature limit will behave somewhat differently. Finally, we also discuss the Krylov complexity (entropy) in the symmetry breaking phase with the Higgs scalar field at finite temperatures.