An online measurement device for crystal size and shape in a high-solid-content crystallization process includes a solution amplifier, a measurement device, a peristaltic pump, a crystallization kettle, a dilution device and a solution storage tank. A crystal-containing solution is arranged in the crystallization kettle; an inner wall of the solution amplifier is smooth, one end is an amplification end, and the other end is a contraction end. The contraction end is communicated with one end of the solution storage tank and one end of the crystallization kettle. The amplification end is communicated with the dilution device and the peristaltic pump. The peristaltic pump is communicated with the other end of the crystallization kettle. The solution amplifier, the peristaltic pump and the crystallization kettle form a complete passage through a pipeline. A measurement instrument of the measurement device is arranged at the outer side of the solution amplifier.