Abstract: SIM is an advanced interactivc mixed-level logic simulatipn tool, one of the important parts of the VLSI CAD system PANDA. SIM can simulate gate-level, functional-block-level and switch-level circuits,and their mixture.In gate-level and functional-block-level, the simulation uses all kinds of delay types,A functional block can be defined by users using the function description language FDL.In the switch-level simulation we use MOSSIM-based algorithm to calculate steady state value.Instead of the unit delay used in MOSSIM Algorithm, we use nodal delay poteutial equations, combining with MOS transistors equivalent dynamic resistance, as well as the effect of the slope of the input waveforms, to calculate the signal delay, so the calculation accuracy is much better than that of gate-level. SIM has convenient interactive ability to help the users to operate the tool.During the developing time, we obey software engineering method. This paper introduces its basic functions and total design ideas.