In practical construction， there are usually some foundation pits with large scale， long construction period and are overwintering. Therefore， the work of proper cold-insulation and protection needs to be done. Based on a deep foundation pit project of a building in Ulaanbaatar， the capital of Mongolia， this paper studies the cold-insulation and protection technology for safety wintering of deep foundation pits. B2 grade polystyrene board is used to insulate and prevent freezing of the soil at the bottom of the pit. Greenhouse was built to keep the leakage area on the side wall of the foundation pit warm. Measures of electric heat tracing and insulation with rock wool are adopted to ensure normal dewatering of the foundation pit. The actual construction situation confirmed the effectiveness of the above technology， which can provide reference for the insulation and protection of safe overwintering of foundation pit projects in the future.