The invention discloses a real-time on-line detection system for tension of a steel wire rope for an elevator. The detection system comprises a cloud server, a sensor, an information collection board, an intelligent decision-making terminal, a hand-held monitor and an elevator system, wherein the sensor monitors the tension for dragging the steel wire rope in real time and online displays a numerical value of the tension received by the steel wire rope, so that all the tension for dragging the steel wire rope is conveniently adjusted to be uniform, the service life of the steel wire rope is prolonged, and the elevator running comfortableness is improved; by monitoring the change of the numerical value of the tension for dragging the steel wire rope in real time, whether there are persons in a car can be determined, and a weighing function of the elevator is realized; the intelligent decision-making terminal receives data, makes a local decision according to set logic, and controls the elevator to emergently stop running or stop running after the elevator stops nearby in an abnormal state, thereby avoiding sudden accidents; and the cloud server collects historical big data, performs comparison, analysis, judgment and depicts a change trend of the tension of the steel wire rope, and if the trend becomes bad, a preventive and targeted maintenance service can be performed, thereby improving a safety coefficient of elevator running.