Rapid extraction and screening of high-purity DNA fragments is an indispensable technology in advanced molecular biology. In this article, mesoporous magnetic composite microspheres (MSP@mTiO₂) with tunable pore sizes were successfully fabricated for high-purity DNA extraction and fragment screening. Owing to the strong complexation ability of Ti ions with DNA phosphate groups and the high specific surface area of mesoporous microspheres, the MSP@mTiO₂ microspheres possess excellent adsorption performance, where the saturated loading capacity of MSP@mTiO₂ with a specific surface area of 122 m² g⁻¹ is as high as 575 μg mg⁻¹ for a salmon sperm specimen. ITC experiments demonstrated that DNA adsorption on MSP@mTiO₂ microspheres is mainly driven by entropy, which gives us more potential ways to regulate the balance of adsorption and desorption. Meanwhile, the mesoporous MSP@mTiO₂ microspheres exhibit a much higher extraction efficiency compared with non-porous MSP@TiO₂ for whole genome DNA from Arabidopsis thaliana plants. Interestingly, DNA fragments with different lengths could be screened by simply regulating the pore size of MSP@mTiO₂ or the concentration of Na₃PO₄ in the eluent. A small pore size and low phosphate concentration are advantageous for the extraction of short-stranded DNA fragments, and DNA fragments (≤1000 bp) can be efficiently extracted when the mesopore size of MSP@mTiO₂ is lower than 7.6 nm. The extraction results from the mesoporous composite microspheres provide new promising insights into the purification and screening of DNA from complex biological samples.