The invention discloses a system for performing online measurement on the sagging degree of a thin film and immediately performing improvement on the sagging of the thin film according to a measurement result. With the system, on-line measurement and improvement can be performed on all width ranges of the thin film; and a time interval is provided for collection of the on-line measurement data. Through the measurement and improvement system, the sagging degree is reduced by 15%-50% compared with the condition that the sagging degree is not subjected to the system. Meanwhile, the invention further discloses a battery diaphragm. The sagging degree is smaller than or equal to 100 mm when the measurement system is adopted for measurement, and a difference value of the sagging degree between two points in any length direction on a fixed width position is smaller than or equal to 50/1000 mm.