The invention provides a redundant controlled micro underwater robot and a fault diagnosis and fault-tolerant control method. A shell has a streamline shape spliced by a lower shell, an upper front shell and an upper back shell; an inclination compass cabin, a depth gauge, a battery tank, a Doppler speed meter, an electronic cabin, horizontal propeller impellers and a turning rise-sink control actuator are arranged inside the shell from the front to the back; and the depth gauge, the battery tank and the Doppler speed meter are arranged in parallel and positioned at the back of the inclination compass cabin together, the turning rise-sink control actuator is positioned in the middle of two horizontal propeller impellers, and the two horizontal propeller impellers and the turning rise-sink control actuator are arranged transversely in parallel and positioned at the tail of the underwater robot together. The robot and the method have the characteristics of high intellectualization degree, strong fault-tolerant capability, good safety, flexile motion, and convenient structural adjustment.