Abstract: The measurement of electron beam envelope at different position in the anode pipe of injector with a large area thermionic cathode for a linear induction accelerator is very important. It can help to know the emission characteristic of the thermionic cathode for multi pulsed beam and to debug a matched magnetic field for the electron beam transmission in the anode pipe. Dragon-Ⅱ injector adopts a large area thermionic cathode. The period time for heating up or cooling down is very long and the demand of vacuum in the injector is very high especially during operation. It is a problem to destroy the vacuum frequently in order to measure the electron beam envelope in the injector with some ready-made measurement. A multi pulsed electron beam envelope measurement system mainly consists of an extendible and retractile sleeve and a high speed multi framing camera and an optical transition radiation target has been developed to achieve the measurement in the injector without destroying the vacuum. This system can measure envelope at different position in the whole range of the anode pipe and obtain some time-resolved results. This measurement ability has improved greatly the debugging efficiency of Dragon-Ⅱ injector.