The invention relates to a large slenderness ratio propellant online detection device, and belongs to the machinery electronic automation field; according to grain, dimension and weight requirements,the invention designs a single-station online detection device mainly comprising a pedestal, a sucker and a connecting rod, a screw rod, a linear displacement driving motor, a rotation displacement driving motor, a code reading system, a weight sensor, a diameter sensor and a length sensor; according to theory and simulation verifications, the detection device can detect the grain, dimension and weight information while ensuring the safety and product quality; the detection process is continuous, automatic, and man-machine isolated, thus improving the production efficiency, the intrinsic safety and the product quality.