A method for preparing microporous PVA fiber comprising the following steps: Step 1: preparing a spinning solution: mixing raw material PVA resin with water and heating to dissolve the raw material PVA resin completely to form the spinning solution; preparing calcium hydroxide solution: mixing calcium hydroxide powder with water to form the calcium hydroxide solution; preparing sodium sulfate solution: forming the sodium sulfate solution with a concentration of 35% from mirabilite and water; Step 2: preparing PVA spinning stock solution: cooling the spinning solution obtained in Step 1 to 40-60°C, and adding a foaming agent thereto to provide the PVA spinning stock solution; Step 3: spinning and first coagulation bath treatment: spinning the PVA spinning stock solution obtained in Step 2, and subjecting the spinned fiber to a first coagulation bath treatment in the sodium sulfate solution for dehydration to provide a primary PVA fiber; Step 4: second coagulation bath treatment: reacting the primary PVA fiber obtained in Step 3 with the calcium hydroxide solution to provide a secondary fiber; Step 5: foaming and pore forming: heating to foam the secondary fiber obtained in Step 4 while stretching to form a primary product of microporous PVA fiber; and Step 6: cleaning and drying the primary products of microporous PVA fiber obtained in Step 5 to provide the final product of microporous PVA fiber; the foaming agent is one of ammonium carbonate and ammonium bicarbonate, in Step 1, the weight ratio of water to calcium hydroxide is 1: (0.006-0.02), and in Step 2, the weight ratio of the PVA resin to the foaming agent is 1: (0.0006-0.0009). Step 1: preparing a spinning solution: mixing raw material PVA resin with water and heating to dissolve the raw material PVA resin completely to form the spinning solution; preparing calcium hydroxide solution: mixing calcium hydroxide powder with water to form the calcium hydroxide solution; preparing sodium sulfate solution: forming the sodium sulfate solution with a concentration of 35% from mirabilite and water; Step 2: preparing PVA spinning stock solution: cooling the spinning solution obtained in Step 1 to 40-60°C, and adding a foaming agent thereto to provide the PVA spinning stock solution; Step 3: spinning and first coagulation bath treatment: spinning the PVA spinning stock solution obtained in Step 2, and subjecting the spinned fiber to a first coagulation bath treatment in the sodium sulfate solution for dehydration to provide a primary PVA fiber; Step 4: second coagulation bath treatment: reacting the primary PVA fiber obtained in Step 3 with the calcium hydroxide solution to provide a secondary fiber; Step 5: foaming and pore forming: heating to foam the secondary fiber obtained in Step 4 while stretching to form a primary product of microporous PVA fiber; and Step 6: cleaning and drying the primary products of microporous PVA fiber obtained in Step 5 to provide the final product of microporous PVA fiber; the foaming agent is one of ammonium carbonate and ammonium bicarbonate, in Step 1, the weight ratio of water to calcium hydroxide is 1: (0.006-0.02), and in Step 2, the weight ratio of the PVA resin to the foaming agent is 1: (0.0006-0.0009).