The invention discloses an on-line scale prevention and removal agent for circulating water, belongs to the technical field of scale removal agents, and effectively solves the problem that an existing circulating water system cannot be integrally cleaned in the production and operation process. Comprising the following raw materials in parts by weight: 30-50 parts of polydiamino-terminated sodium acrylate, 10-30 parts of deionized water, 1-2 parts of an imidazoline surfactant, 1-2 parts of a pH regulator, 1-5 parts of a corrosion inhibitor, 1-5 parts of a chelating agent, 5-10 parts of a dissolving agent and 5-10 parts of a cosolvent. The prepared polydiamino-terminated sodium acrylate is used as a main raw material, descaling operation of a circulating water system can be carried out under the condition that normal operation of the system is not affected, the descaling effect is good, the efficiency is high, meanwhile, by adding the imidazoline type surfactant, the corrosion inhibitor, the chelating agent and the cosolvent, the descaling effect is good, and the descaling effect is good. The scale remover is simple in structure and convenient to use, can effectively prevent calcium and magnesium ions from reacting to form precipitates in the descaling process, avoids the phenomenon that dirt blocks a pipe, is low in cost, and has good economic benefits and wide market prospects.