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How statistical behavior of semiflexible polymer chains may be affected by force stretching and tube confinement is a classical unsolved problem in polymer physics.Based on the Odijk deflection theory and normal mode decomposition in terms of Fourier expansion,we derived a new compact formula on the extension of the wormlike chain of finite length strongly confined in a tube and simultaneously stretched by an external force.We also suggested a new deflection length,which together with the force-extension relation is valid for a much extended range of the tube-diameter/persistence-length ratio comparing to the classic Odijk theory.The newly derived formula has no adjustable fitting parameters for the whole deflection regime,in contrast,the classic Odijk length needs different prefactors to fit the free energy and average extension,respectively.Brownian dynamics simulations based on the generalized bead-rod (GBR) model were extensively performed,which justified the theoretical predictions.