Downhole operation well fluid balance buffering method and multifunctional buffering fluid valve

The invention discloses a downhole operation well fluid balance buffering method and a multifunctional buffering fluid valve applicable to multiple processes of oilfield downhole operation field construction. According to the downhole operation well fluid balance buffering method, one buffering fluid valve is connected to the upper portion of a drift size gauge tool or other downhole operation tool in a downhole operation string, speed of well fluid entering the inside of an oil pipe in the process of lowering of the down-hole string is effectively controlled, fluid level in the oil pipe is allowed to be lower than that in a casing, pressure in the oil pipe is enabled to be basically kept in balance with the pressure in the casing, and the well fluid is prevented from spattering from the oil pipe to the ground. The fluid valve comprises a working barrel connected with a pressing cap, and a bidirectional fluid control device is mounted in the working barrel. The fluid levels in a shaft can be kept in balance, the oil pipe is prevented from injection and overflow in the lowering process of the oil pipe string, cleanliness and sanitation are realized, and operation is facilitated. Meanwhile, requirements on conventional and inverse well flushing are met, slurry is ensured to be displaced completely, an oil layer is protected against secondary damage, and technical problems perplexing production for a long time are solved.