The invention provides an online monitoring system for abrasion quantity and corrosion quantity of a flue gas side of a heating surface. The online monitoring system comprises a film-coated sensor, wherein the film-coated sensor is formed by a plane carrier base, a leading wire and a thin film resistor made from a material which is the same as the material of the heating surface; the thin film resistor is uniformly arranged on the plane carrier base; the plane carrier base is fixed to the heating surface; the thin film resistor is connected with a resistor measurement system through a guide wire. According to the online monitoring system provided by the invention, the defects of the prior art are overcome, the film-coated sensor is combined with the high-temperature heating surface, the resistance change of the film-coated sensor can be measured by adopting a high-accuracy data collector, and the change of resistance thickness can be reversely calculated and obtained according to a relationship between the resistance thickness and a resistance value, so that the change of the abrasion quantity and the corrosion quantity of the flue gas side of the heating surface can be obtained; the online monitoring system is simple in structure, low in cost and convenient and accurate in measurement, is beneficial for safe and stable running of the heating surface, and is capable of reducing unplanned outage of a unit caused by abrasion and corrosion of the heating surface.