Disclosed is a method of improving HC emission when an engine is cold-started. The method includes the following main steps: 1. fresh air is injected into a passage behind an exhaust valve of the engine; 2. when the exhaust valve is opened, the fresh air performs oxidation reaction directly with exhaust gas; 3. the exhaust gas enters a three-way catalytic converter for catalyzing and converting. A system realizing the method comprises an inlet of a secondary air valve which connects an electromagnetic valve control and a secondary air pump extracting air from an air filter, an oxygen sensor, the three-way catalytic converter and an electronic control unit; wherein the rear pipe of the exhaust valve of the engine is connected with the three-way catalytic converter, the oxygen sensor is arranged before the three-way catalytic converter, and the electronic control unit is connected with the electromagnetic valve and the oxygen sensor via electricity. The method is characterized in that the rear outlet of the secondary air valve is connected with the exhaust valve of the engine. Since the position where secondary air is injected into the exhaust gas is arranged directly behind the exhaust valve, the oxidation reaction utilizes the temperature of the exhaust gas to the greatest extent, emission of HC and Co is greatly reduced, and the catalytic converter is started as soon as possible, thereby resulting in that emission requirements of cold-started emission values can be achieved more easily.