A key coding circuit includes N keys, a signal processing unit, a bus, and a ground wire, and each key includes an input end and a ground end electrically connected to the ground wire. The signal processing unit includes M signal lines, where M is the smallest positive whole number not less than Log(N+1, 2), and at least one signal line is electrically connected to the corresponding input end. Each key is coded for M-bit binary, the floating signal lines correspond to the binary code 1, and the signal lines corresponding to binary code 0 are electrically connected to the input end of the corresponding keys.