Theoretical equations and a calculation method giving cloud point curve (CPC) for a multicomponent polymer 1/multicomponent polymer 2/single solvent system (i.e. quasi‐ternary system) were derived on the basis of the Flory‐Huggins solution theory. The systematic calculations based on this method were carried out to examine the effects on CPC of the type of molecular weight distribution, the polydispersity and the weight‐average degree of polymerisation of the two original polymers, and three thermodynamic interaction parameters between solvent‐polymer 1 (X01), solvent‐polymer 2 (X02) and polymer 1‐polymer 2 (X12). The shape of CPC sensitively depends on the polydispersity of the original polymer and, with an increase in polydispersity, the immiscibility region becomes wider.