Solid strong base systems of g-C3N4 by inducing potassium (KC3N4) were constructed for photocatalytically decomposing HCHO. Characterization and theoretical calculation revealed that the formation of K-C and K-N perfected the pi-conjugated structure of g-C3N4, which greatly enhanced the basicity, improve optical and electrical properties of g-C3N4. In the photocatalytic process, K-improved pi-conjugated structure promoted HCHO adsorption, activation, completely oxidative decomposition under visible-light irradiation. Therefore, KC3N4 photocatalysts presented high efficiency in photocatalytic elimination of HCHO, and the optimal apparent rate reached 0.21 mol(-0.17) L-0.17 min(-1) with 1.17 reaction order by promoting the reaction between active oxygen (OH and center dot OH) and formate species under visible-light irradiation, which was 30 times of that (0.007 mol(-0.17) L-0.17 min(-1)) over pristine g-C3N4. These results may suggest a new way for probing the advanced solid strong base photocatalysts for eliminating volatile organic compounds.