According to the needs of model-based assembly in virtual experiments, at the base of making full use of Cult3D, applying Java to expand the function of Cult3D was proposed to realize the interactive model assembly in virtual experiment. The paper analyses the mode of collision detection in procedure of assembling models in virtual experiment, the correspondence between Java and Cult3D and the application of Java multithreading in virtual assembly, which are key technologies for virtual laboratory. Then an example of water tower simulation teaching experiment is supplied.