The present application is applicable to the technical field of vehicle diagnosis, and provides a method and a system for graphically displaying a data stream and a vehicle diagnostic device. The method comprises: receiving and storing diagnostic data; graphically displaying the diagnostic data within a display range according to the display range set by a user; and receiving a zoom-in instruction or a zoom-out instruction sent by the user, zooming in or zooming out the display range according to a preset condition, and graphically displaying the diagnostic data within a zoomed in or zoomed out display range. In this process, the user can randomly zoom in and zoom out a displayed graph, which helps the user to comparatively look over different diagnostic data received within a long period of time so as to rapidly discover a fault or find out a fault cause.