The invention relates to a method and a device for generating a true random number. The device comprises a high frequency oscillator, an initial seed number generation unit, an error diffusion calculation unit, a linear feedback unit, a control unit and a random number output unit, wherein the output end of the high frequency oscillator is connected with the input end of the initial seed number generation unit; the output end of the initial seed number generation unit is connected with the input end of the error diffusion calculation unit; the output end of the error diffusion calculation unit is connected with the linear feedback unit; the output end of the linear feedback unit is connected with the control unit; and the control unit is connected with the random number output unit. The device achieves a metastable state circuit only by using general logic hardware, so the hardware structure is simplified; one-time-pad is realized by storing an initial seed number in a cache reset at each time; judgment and feedback control are realized by using a generator polynomial; and a secure hash algorithm is realized by using an error diffusion process, therefore, an effect that the random number is difficult to copy or forecast is achieved.