The effects of 2 phosphonic butane 1,2,4 tricarboxylic acid(PBTCA), hydroxyethyl diphosphonic acid(HEDP) and diethylenetriamine pentamethyl phosphonic acid(DTPMPA) on the hydration and hardening characteristics of cement paste were studied by ion chelation reaction and measurement of hydration heat and resistivity. The mechanism of it was discussed. The results show that the organic phosphonic acid has drastic chelation effect on calcium ion. The more the single molecule phosphonic acid groups, the greater the chelating value of metal ions, the order of strength from strong to weak are successively DTPMPA, HEDP and PBTCA. The retardation efficiency of the organic phosphonic acid at the same dosage is much higher than that of sodium gluconate, and its inhibition ability against hydration of cement slurry increases with the increase of the number of phosphine carboxyl groups. Organic phosphonic acid preferentially forms stable chelates with calcium and magnesium plasma, which restricts participation of calcium and magnesium plasma in the formation of hydration products, and extends the hydration dissolution and induction period, related to the number of phosphonic groups. Phosphonic acid makes the resistivity differential curve of the hydration accelerated period to show that a pair of peaks and troughs disapear, to be different from the control sample. In addition, the inflexion point of resistivity differential curve is closely related to cement slurry setting time. Appropriate amount of the above organic phosphonic acid has an enhanced effect on the compressive strength and flexural strength of cement 28d, but an excessive amount leads to a significant decrease in strength.