The invention relates to the technical field of nuclear power plants and particularly relates to a pressure sealing method of leakage in a primary loop small-diameter valve of a nuclear power plant. The method is capable of completing pressure sealing of the leakage in the primary loop small-diameter valve under the conditions that halt and shutdown can be avoided and the primary loop does not need to be isolated and emptied; the leakage rate of the primary loop is effectively reduced; normal and safe production and operation of a unit are maintained; halt and shutdown are not required and the primary loop does not need to be isolated and emptied in the maintenance process; the normal operation of the unit is not affected; costs of manpower and material resources are reduced; the maintenance period is shortened; the pressure sealing method has high safety and economic benefit.