A diaphragm, which is taken as one of four materials of a lithium battery, gets a large development opportunity, and simultaneously, more higher requirements to performance consistency and appearance flatness are put forward. Quality of the diaphragm directly influences usage performance and safety performance of the lithium battery. A wave and a tippet are key problem points which influence the appearance flatness of the diaphragm. Each diaphragm manufacturer finds a method to overcome the above techncial problem. In a lithium battery diaphragm surface flatness detection apparatus and a method, a unreeling shaft, an infrared detector, a hanging counterweight, a horizontal base and two guide rollers are included. The detection apparatus possesses characteristics that the detection is simple; an operation flow is easy; efficiency high and so on. The surface flatness of the diaphragm can be embodied through a specific value and evaluation is standardized.