The invention discloses an oil and gas mixing device for start-up of a pulverized coal fired boiler. When the boiler start-up is implemented through the oil, the oil in an oil supply device is sprayed into a combustion chamber through an oil gun, the air in an air source enters the combustion chamber through an air conveying chamber body, the oil mist in the combustion chamber is ignited through an ignition source and the flames are generated by the combustion of the oil mist, and the flames are sprayed out of a flame outlet of the combustion chamber to enter a second air opening of the pulverized coal fired boiler to ignite the pulverized coal. When the oil is in sufficient and needs to be replaced by the gas to implement the boiler start-up, only a gas source needs to be opened, the gas enters the combustion chamber to replace the oil to be burnt to generate flames, and the flames enter the second air opening of the pulverized coal fired boiler to ignite the pulverized coal. When the oil is in sufficient and needs to be replaced by the gas, a device for starting the boiler does not need to be replaced and only the gas source needs to be opened to supply the gas for the combustion chamber for combustion. Accordingly, the device for starting the boiler does not need to be replaced when different fuels are replaced and accordingly the start-up on the boiler can be implemented and the use is convenient and rapid.