The two stage control system, with 486 microcomputer as the master and programmable controller as the slave, is studied. The master communicates with the slave through RS 232C asynchronous series communication interface to realize logical sequence control of extrusion process, control of parison shape and thickness, control of hollow plastic container blow molding,etc. The basic principle of hollow blow molding, the constitution of control system and the construction of intermittent parison thickness controller are described, and the series communication program flow charts between master and slave are presented. Finally, the discussions about the soft hardware debugging and the system expansibility are made.