This paper mainly studies the separation mechanism of a square cyclone at high inlet solid concentration. It is shown that the separation mechanism is very different from that at low inlet solid concentration. A phenomenon called two-instantaneous separation exists, which has never been found in square cyclone experiments before. This phenomenon has bearing on the limit load of gas-solid flow, and the part exceeding the limit load will be separated instantaneously after entering the square cyclone. A model called two-instantaneous separation is introduced in this paper as well as the calculations of limit load and separation efficiency, which can be used in the design and research on a square cyclone for circulating fluidized bed boiler.