The invention relates to a selective alarm based on magnetorheological fluid, which is used for keeping order in a playground for children and ensuring safety of the children. The device is composed of a flat plate, a sealing circle, a shaft frame, the magnetorheological fluid, electromagnetic coins, an isolator, volume compensators, delay switching contacts, reset springs and a shell. When the electromagnetic coins are energized, magnetic circuits are generated in the shell, the shaft frame and the gap, and thus the magnetorheological fluid is converted to a high-viscosity and low-liquidity Bingham fluid from the original Newton fluid. The shaft frame is fixed inside the shell. When enough pressure (larger than the shearing force of the magnetorheological fluid) is applied to the flat plate, the shaft frame begins to move downwardly until to be contacted with the delay switching contact, and an alarm is given. Through adjusting the magnitude of current passing through the electromagnetic coins, the shearing force of the magnetorheological fluid can enable a child to pass through without giving an alarm and an alarm is given when an adult passes. The device selectively gives an alarm through changing the magnitude of the current, and intelligence of the alarm can be realized.