Abstract: The hydrate inhibition and low-temperature stability of formate based drilling fluids，as well as effect of hydrates inhibitors on the fluids，are evaluated elementarily by the experimental test.The results show that a basic system（base liquid）of formate based drilling fluids，which are composed of seawater，3.0% HCOONa，3.0%SK-2，0.2%KPAM，0.1%LV-PAC，2.0%SMP-2 and 0.3% modified starch，has a good performance stability under low temperature environment （-5℃~20℃）.Adding NaCl and KCl，thermodynamic inhibitors，has little effect on the rheology properties of the basic system，while PVP （K30），a dynamic inhibitor，has a great effect on the properties.The basic system has a low ability of hydrates inhibition when it is used in deep water over 1400m.Low dosage of PVP （K30） can increasingly improve the hydrate inhibition of the basic system，and the suitable concentration is about 0.5%.Therefore，the formate based drilling fluids which contain high concentration of salts and low dosage of hydrate inhibitors can meet the needs of gas hydrate drilling in the deep water.

Key words: gas hydrate, drilling fluids, formate, low temperature, inhibitor