The invention discloses a method for in-situ preparation of a boron-contained magnesium base composite material. A groove is formed in a magnesium base alloy matrix plate as a composite material preparation area; the magnesium base alloy matrix plate to be machined is fixed on a stirring friction machining equipment platform by a fixture; boron-contained high-energy ball-milled powder prepared beforehand is uniformly filled in the groove of the plate for compaction; a needle-free stirring head is used for pretreatment in the groove filled with the powder; the powder is sealed in the groove; a needle stirring head matched with the groove in size is used for performing multiple times of stirring friction machining for the powder filling area on the matrix; and the boron-contained magnesium base composite material is obtained by heat generated in the stirring friction process and acute material flowing. The method organically combines the advantages of high-energy ball milling and the stirring friction machining technology to prepare the boron-contained magnesium base composite material, reduces the grain size of the magnesium substrate when effectively obtaining a nanometer size reinforcement phase, and obtains the magnesium base composite material with better comprehensive performance.