Mass loading and potential plateau are the two most important issues of potassium (K)-ion batteries (KIBs), but they have long been ignored in previous studies. Herein, we report a simple and scalable method to fabricate acidized carbon clothes (A-CC) as high mass loading (13.1 mg cm) anode for KIBs, which achieved a reversible areal-specific capacity of 1.81 mAh cm at 0.2 mA cm. Besides, we have proposed the concept of “relative energy density” to reasonably evaluate the electrochemical performance of the anode. According to our calculation method, the A-CC electrode exhibited an ultrahigh relative energy density of 46 Wh m in the initial charge process and remained at 40 Wh m after 50 cycles. Furthermore, we performed the operando Raman spectroscopy (ORS) to investigate the K-ion storage mechanism. We believe that our work might provide a new guideline for the evaluation of anode performance, thereby, opening an avenue for the development of commercial anode.