The invention relates to a long-term preservation method for a well core, which mainly solves the problem of difficult preservation because a fractured and broken core taken out of a shaft bottom is difficult to cut or difficultly forms the entire plane after being cut. The method is characterized by comprising the following steps of: preparing a casting material; putting the core to be preserved into a U-shaped polyethylene support groove; slowly pouring the casting material into the U-shaped polyethylene support groove filled with the core; and sizing gel at room temperature, cutting the core after solidifying and sizing in a thermostatic drying oven, putting the core into a U-shaped transparent organic glass support groove for secondary casting and smoothing, grinding and polishing the surface of crystal gel in the U-shaped transparent organic glass support groove until the crystal gel is bright and transparent. By the method, the core can be integrally cut, a cut plane is smooth, the original core is preserved permanently and cannot be broken and the core is convenient to observe in all dimensions.