Programmable logic controllers (PLCs) are widely used in industrial applications. Ensuring the correctness of PLC programs is important due to their safety-critical nature. Structured text (ST) is an imperative programming language for PLC. Despite recent advances in executable semantics of PLC ST, existing methods neglect complex multitasking and preemption features. This paper presents an executable semantics of PLC ST with preemptive multitasking. Formal analysis of multitasking programs experiences the state explosion problem. To mitigate this problem, this paper also proposes state space reduction techniques for model checking multitask PLC ST programs.