The invention relates to a method for improving low-speed performance of a multi-cylinder engine. The method comprises a process of keeping an engine valve open in constant by using a solid-chained mechanism. In the first predetermined engine speed, a working piston of the solid-chained mechanism is placed on the operating position, forming the mechanical link with the engine valve, keeping the opened engine valve from closing; and in the first predetermined engine speed or the lower engine speed including zero, keeping the engine valve open in constant. Before the engine start and during the start, the working piston of the solid-chained mechanism keeps the engine valve open in constant sequentially, until the engine speed is increased from zero to the second predetermined engine speed, the working piston is switched from the operating position to the non-operating position, and cancelling the effect on the engine valve. The method uses the novel method and mechanism, and is capable of keeping an inlet valve or an exhaust valve of the engine open in constant, improving the parking, starting and idling performance of the engine, and reducing the fuel consumption and discharge of the engine.