Petri box calculus (PBC) is a flexible and expressive process algebra defined by E. Best, R. Devillers and others in 1992. In this paper, we present the extension of discrete time stochastic Petri box calculus(dtsPBC) presented by I.V. Tarasyuk in 2007 with immediate multiactions, which is a discrete time analog of stochastic Petri box calculus (sPBC) with immediate multiactions proposed by H. Maci`a, V. Valero and others in 2008 within a continuous time domain. The step operational semantics of the calculus is constructed via labeled probabilistic transition systems. The denotational semantics is defined on the basis of a subclass of labeled discrete time stochastic Petri nets with immediate transitions. A consistency of both semanticsis demonstrated. In order to evaluate performance, the corresponding stochastic process is analyzed, which is the same for both semantics. A case study of the shared memory system is presented as an example of specification, modeling, behaviour analysis and performance evaluation for parallel systems.