Abstract: A paper feeding device includes a pickup mechanism, a conveying mechanism disposed behind the pickup mechanism, a separation mechanism disposed behind the conveying mechanism, a feeding mechanism disposed behind the separation mechanism, a transmission mechanism driving the pickup mechanism, the conveying mechanism, the separation mechanism and the feeding mechanism, and an ultrasonic circuit unit disposed between the separation mechanism and the transmission mechanism. The ultrasonic circuit unit includes an ultrasonic detecting module, an amplifying circuit unit electrically connected with the ultrasonic detecting module, a paper density detecting unit electrically connected with the amplifying circuit unit, an analog-to-digital converter electrically connected with the paper density detecting unit, a processor electrically connected with the analog-to-digital converter, and a driving unit electrically connected with the processor and the transmission mechanism.