The invention relates to a new technique method for magnetic powder detection of a large cast valve, which comprises the following steps: 1), processing the surface of the valve body; 2), uniformly spraying contrast sensitivity on the surface of the valve body; 3), detecting whether the equipment is in the check validity period or not; 4), configuring a magnetic suspending liquid with the concentration within 10-25g/L; 5), fixing a Al-30/100 sensitivity test piece in a detected part through a standard test piece fixing device so as to assess the comprehensive performance of a detecting system; 6), wetting the surface of the workpiece by the magnetic suspending liquid, magnetizing the surface of the workpiece, and detecting the valve body according to a special magnetized path graph for the valve body inspection via an AC magnetic yoke continuous method; 7), utilizing an air pressure type sprinkling can with a lighting function and suitable for magnetic powder detection, directly performing observation once magnetic marks are formed, and achieving that magnetization, magnetic suspending liquid spraying and observation are performed at the same time; 8), recording detection parameters and defect magnetic marks, grading the workpiece quality according to the detection technique standard based on the displayed defect magnetic marks, and then determining whether the workpiece is qualified or not according to the acceptance quality level. Moreover, in case of reinspection, the sensitivity of the detecting system is readjusted, and the workpiece is detected again until the requirements are met. The method can be effectively used for detecting the magnetic powder of the large valve.