Calcium phosphate (CaP) nanoparticles present a class of promising nonviral gene delivery carriers due to their easy-preparation procedure, low toxicity and high transfection efficiency. PEGylated polyanion block copolymer was widely used to control the CaP crystal growth during CaP co-precipitation with therapeutic genes. However, there also exists a competition between the entrapped gene and polyanion in the system, which results in reduced stability and low gene entrapment efficiency. To solve this problem, herein, we demonstrate a non-anion stabilization strategy using a boroxole-containing block polymer, PEG-b-poly(benzoxaborole) (PEG-PBO), which forms pH-responsive boronic ester bonds with ribose rings of siRNA and also excellently adhers to the hydroxyapatite surface of CaP. The PEG-PBO/siRNA/CaP nanocomposites exhibited high siRNA loading efficiency, low cell cytotoxicity and excellent colloidal stability at neutral pH. The nanocomposites easily entered cancer cells mainly via clathrin-mediated endocytosis and transferred into acidic lysosomes, where the boronic esters broke, nanoparticles dissociated, and siRNA released and escaped from lysosomes. PEG-PBO/siRNA/CaP nanocomposites showed significantly higher gene silencing efficacy than lipofectamine 2000/siRNA lipoplex in multiple cancer cells. Thus, PEG-PBO/siRNA/CaP nanocomposites possess potential for safe and effective siRNA delivery.