Abstract: During deepwater cementing, early strength of oil well cement grows slowly. To solve this problem, the cryogenic early strength behavior of nano-SiO2 was investigated. By means of progressive superposition optimization, a nano-SiO2 complex accelerator NS-4 was obtained. The effects of this accelerator on the thickening performance, static gel strength and compressive strength were analyzed under simulated temperature and pressure conditions in deepwater cementing. The nano-SiO2 complex accelerator composed of 0.8% nano-SiO2 (mass fraction, the same below) + 2% sodium sulfate + 0.05% triethanolamine + 0.2% sodium aluminate is adopted,with its volume added being 3.05% of the cement volume. The test results show that, under the conditions of 15℃ and 10 MPa, the thickening time of cement slurry mixed with this accelerator is 236 min, including 34 min thickening transition time. Under the conditions of 30℃ and 15 MPa, the development time of 0-576 Pa static gel strength is 180 min, including 48 min gelling transition time. Under the conditions of 4℃, 15℃ and 30℃, the 24 h compressive strength of hardened cement mixed with this accelerator is 9 times, 4.7 times and 2.4 times the pure G cement respectively. The excellent cryogenic early strength and gas blocking performance of the accelerator contribute to solving such problems as low temperature and superficial water/gas flow confronted in deepwater cementing.