To satisfy the practical needs of the sustainability and industrialization of supercapacitors, it is essential to maintain good electrochemical performance at high mass loading above the commercial levels (10 mg cm−2). Herein, a unique 3D porous carbon with high mass loading is successfully prepared from Linum usitatissimum L. stem via a simple route of high temperature carbonization and subsequent KOH activation. The as-prepared porous carbon displays rich N/O heteroatoms, hierarchical porous, and interconnected vertical channels, which enable fast electron/ion transport and afford additional capacitance. These characteristics give the resultant carbon high capacitances of 434/245 F g−1 at 1/50 A g−1. Importantly, the as-prepared carbon can achieve mass loadings of 20 mg cm−2 and maintain decent electrochemical performance at this mass loadings, owing to short electron/ion transfer pathway as well as sufficient active sites. Specifically, the assembled symmetric supercapacitor delivers superior areal capacitance of 3.7 F cm−1 at 1 A g−1 and remarkably capacitance retention of 2.5 F cm−1 at 5 A g−1 under 20 mg cm−1. This work enlightens a sustainable and facile way for constructure advanced electrode material.