The invention discloses a paper window forming device for a paper bag machine. The paper window forming device can be used by being combined into the paper bag machine in a module mode and can also separately form a window in roll paper through an independent transmission mechanism, and the effect of one machine with multiple purposes is achieved; a tool is fixed in a magnetic installation mode, installing is quick, and the tool is replaced conveniently; window waste is separated from the paper through the pneumatic principle and fed into a waste tank to be stored; a window forming position is adjusted through a bottom adjusting mechanism in the state that the machine is not stopped; the window manufacturing speed does not affect the normal working speed of paper bags, the waste can be discharged successfully, and the manufacturing cost is low; and thus, the production efficiency is improved, the quality of the paper bags is ensured, and the wide practicability is achieved.