Urea injection control method and device

The invention provides a urea injection control method and device which are applied to a two-stage SCR aftertreatment system.The method comprises the steps that the first temperature mean value of a preceding-stage SCR, the second temperature mean value of a backward-stage SCR, the first nitrogen oxide measured value of the upstream of the preceding-stage SCR, the second nitrogen oxide measured value of the downstream of the preceding-stage SCR, the exhaust flow and the measured ammonia reserve of the backward-stage SCR are obtained; according to parameters such as a target mode started by the engine, the first temperature mean value and the second temperature mean value, the first set urea injection amount of the preceding-stage SCR is determined; according to parameters such as the exhaust flow and a second nitrogen oxide measured value, the second set urea injection amount of the post-stage SCR is determined; the first set urea injection amount is used for controlling the urea nozzle of the preceding-stage