We propose the quantitative mean-field theory of mechanical unfolding of a globule formed by long flexible homopolymer chain collapsed in poor solvent and subjected to extensional deformation. We demonstrate that depending on the degree of polymerization and solvent quality (quantified by the Flory-Huggins chi parameter) the mechanical unfolding of the collapsed chain either may occur continuously (by passing a sequence of uniformly elongated configurations) or involves intramolecular microphase coexistence of a collapsed and a stretched segment followed by an abrupt unraveling transition. The force-extension curves are obtained and quantitatively compared to our recent results of numerical self-consistent-field (SCF) simulations. The phase diagrams for extended homopolymer chains in poor solvent comprising one- and two-phase regions are calculated for different chain length or/and solvent quality.