The invention discloses a low-heat cement slurry capable of preventing gas hydrate from decomposition. The cement slurry is composed of the following components in parts by weight: 44-70 parts of slurrying water, 5-11 parts of enhancer, 7-16 parts of light-weight additive, 1-3 parts of setting accelerator, 1-6 parts of water-loss control agent, 1-5 parts of dispersing agent, 0.5-2 parts of defoaming agent, 8-16 parts of phase-change energy storage material and 100 parts of cement. The phase-change energy storage material in the low-heat cement slurry can effectively absorb heat released by cement hydration, thereby ensuring the temperature of the hydrate layer oil well cement slurry to be stable and ensuring the temperature of the hydrate layer. The low-heat cement slurry is slightly influenced by low temperature of deep water; and when the cement slurry temperature is lowered, the phase-change energy storage material can release certain heat to ensure the stability of the cement slurry temperature, thereby being beneficial to development of early strength of the hardened cement and successfully solving the problem of difficulty in acquiring favorable early strength of the cement slurry under the condition of deep water low temperature.