Theory of thermal fluctuations in two-band superconductors under an essentially homogeneous magnetic field is developed within the framework of the two-band Ginzburg-Landau theory.The fluctuating specific heat is calculated by using the optimized self-consistent perturbation approach and the results are applied to analyze the thermodynamic data of the iron-based superconductors Ba1-xKxFe2As2 with x ～ 0.4,which have been suggested to have a two-band structure by recent experiments.We estimate the fluctuation strength in this material and find that the specific heat is described well with the Ginzburg number Gi =4·10-4.The influence of interband coupling strength is investigated and the result of the two-band Gaussian approximation approach is compared.