The delivery of nanomedicines into internal areas of solid tumors is a great challenge for the design of chemotherapeutic drugs and the realization of their successful application. Herein, we synthesized stable and efficient selenium nanoparticles (SeNPs) with an ideal size and a transcellular transport capability for the penetration and treatment of a solid tumor, utilizing Tw-80 as a dispersing agent and mushroom polysaccharide lentinan (LET) as a decorator. In vitro cellular experiments demonstrated that this nanosystem, LET-Tw-SeNPs, renders significant cellular uptake of HepG2 by receptor-mediated endocytosis and exhibits predominant transcellular transport and penetration capacity towards HepG2 tumor spheroids. Moreover, this therapeutic agent simultaneously inhibits the proliferation and migration of HepG2 cells via a cell cycle arrest pathway. Internalized LET-Tw-SeNPs give rise to the overproduction of intracellular reactive oxygen species (ROS), thus inducing mitochondrial rupture. Meanwhile, pharmacokinetic analysis showed that LET-Tw-SeNPs displayed a long half-life in blood. Altogether, this study demonstrates an inventive strategy for designing nanosystems with high permeability and low blood clearance, in order to achieve efficient in-depth tumor drug delivery and future clinical treatment of solid tumors.