In this paper, the eco-friendly sodium alginate-silicon oxide capsule (SASC) with a brilliantly wide pH tolerance property was obtained via the one-step titration-gel method and in-situ sol-gel progress. The spherical droplets with SA and tetraethyl orthosilicate (TEOS) casting mixture were directly injected into the calcium chloride (CaCl2) solution. The effects of SiO2 nanoparticles (SiO2 NPs) on the morphology and adsorptive properties of the capsules were investigated. The interface of the SASC with micro-porous fracture aperture and microscopic re-entrant structures help to provide a stronger interaction between MB and surface channel thus increased its removed capacity during the courses of adsorption. The maximum absorption amount of the SASC against methylene blue (MB) was found to be 350.80 mg/g at 25 °C and the solution pH value of 8, which had a 12.82% enhancement compared with the neat capsule. The adsorptive performance of the prepared capsules was literally fitted the pseudo-second-order kinetic equation and Langmuir absorption isothermal model. Moreover, the pH of the MB solution was evidently declined after adsorption, indicating that the adsorption mechanism of the capsules against MB might be dominated by ion-exchange and electrostatic attraction. This facile method which was used to generate free-standing spherical support structures with specialized morphology might be more suitable for removing cationic pollutants from alkaline wastewater.