The necessity of the open architecture for fine-grained parallel model simulating system (WinALT) is discussed. The description of WinALT open architecture and external module interfaces is given. WinALT consists of the language and graphical user's interface subsystems and the kernel. The extensibility of WinALT is implemented in the kernel by a number of interfaces. The principal ones are object file format support interface and those of language and graphical subsystem extensibility. A number of samples is given to clarify the usage of all these interfaces.