The invention relates to a method of monitoring rusting risk of reinforcement in concrete in situ. The method includes the steps of 1, according to the principle of plane state of stress and stress concentration, applying prestress to rusting monitoring sensing units to enable them to be in a plane state of stress; 2, packaging three sides of each rusting monitoring sensing unit, and reserving one side as a rusting monitoring side; 3, distributing the rusting monitoring sensing units at equal intervals within a rectangular steel frame, before concreting, distributing the rectangular steel frame and the outer surface of concrete at a certain angle in an area to be subjected to reinforcement rusting monitoring, allowing the rusting monitoring sides to face a protective layer, and performing rusting monitoring through continuous strain measurement of different sectional heights of sheets. The method has the advantages that monitoring chloride threshold concentration fronts at different depths is achieved, initial rusting time of the reinforcement is predicted, results are updated according to monitoring data, the rusting monitoring sensing units as deep as the reinforcement allow the final initial rusting time, rusting rate and rusting degree of the reinforcement to be directly monitored.