When seawater is used to prepare fracturing fluid，its high salinity and high content of calcium and magnesium ions will affect the swelling，crosslinking and gel breaking of fracturing fluid. In order to reduce the influence of seawater on fracturing fluid，the organophosphonic acid chelating agent SW-CA was prepared from formaldehyde，phosphorous acid and polyvinyl polyamine. The influencing factors of the chelation effect were studied. Crystal morphology was observed by scanning electron microscopy. The mechanism of action was analyzed，and the effect of SW-CA on the crosslinking performance and gel breaking of fracturing fluid was further evaluated. The results showed that the chelating value of SW-CA to Ca2+ and Mg2+ was 276 mg/g and 218 mg/g，respectively. The chelating effect increased with the increase of SW-CA dosage，and decreased with the increase of salinity， temperature and pH value. The precipitated crystal particles became loose and dispersed after adding chelating agent. The mechanism of action was mainly chelating and lattice distortion. The addition of 1% chelating agent SW-CA in seawater could improve the cross-linking performance，viscosity，temperature resistance and shear resistance of base fluid. The fracturing fluid adding 1% SW-CA had controllable gel breaking time and good gel breaking performance. The residual content was 113 mg/L. After soaking ceramsite in fracturing fluid gel breaking liquid with chelating agent for 4 h，only a small amount of guanidine gum residue was adsorbed on the surface of ceramsite，which prevented the precipitation of inorganic salts and reduced the influence on the conductivity of proppant. The chelating agent SW-CA had good compatibility with fracturing fluid，meeting the requirements of on-site construction.