The invention discloses a liquid-propellant rocket engine ignition device and an ignition method thereof. The device comprises an igniter base, a front end socket and a shell. The igniter base is of a T-shaped structure, the two sides of the igniter base are provided with electric ignition tube connectors, and the bottom of the igniter base is connected with the front end socket. The other end of the front end socket is connected to the shell. A gunpowder box is installed in the front end socket. The gunpowder box is a hollow cylindrical gunpowder box with one closed end, the closed end is provided with a plurality of spray holes, and the unclosed end is connected with the front end socket. A gunpowder frame is installed in the shell. A gunpowder column is arranged in the gunpowder frame. A heat insulation layer is arranged between the shell and the gunpowder frame. A heat insulation sleeve is arranged between the gunpowder frame and the gunpowder column. A gunpowder blocking plate is arranged between the bottom of the gunpowder frame and the bottom of the shell. According to the liquid-propellant rocket engine ignition device, the space occupation rate can be significantly reduced, and the structural mass of the liquid-propellant engine is reduced. Many kinds of gunpowder can be selected and are unrelated with an engine supply system, influences on an injector structure are small, and use and maintenance are convenient.