Based on the stability theory of a multicomponent system

the criteria of spinodal and critical point for the mixture consisting of one discrete component and one polydispersed component described in the equation of state are derived If the continuous components are regarded as an intuitive single component

the criteria can be divided into two parts: the first part is the same as the criteria of a binary system

the second one is related to the distribution of polydispersed component The van der Waals equation of state is used to calculate the critical loci of an intuitive system The influence of average molecular weight and its dispersity on the critical phase behavior is examined