This article presents a method for the H-2-norm computation of single-loop finite-dimensional linear continuous periodic systems, which is based on the parametric transfer function concept and the calculus of Fredholm integral equations of the second kind. General formulae for the computation of the mean output variance are provided as relations of two integral functions of an auxiliary parameter. On the basis of the general solution, an approximate computation method is derived, and the deviation of the approximate solution is estimated. In principle, this approach allows one to perform the computation with any desired accuracy.