In this study, an excellent phosphate adsorbent was prepared for removing phosphate excellently to a severe low concentration. The La(OH)3 nanorods stabilizing in polyacrylonitrile (PAN) nanofibers (PLNFs) were designed by electrospinning and a subsequent in situ precipitation. PAN nanofibers were employed as the matrix of the composite nanofibers, where the well-dispersed La(OH)3 nanorods were encapsulated as the active species for highly efficient phosphate capture owing to the strong binding between phosphate and lanthanum. On account of the nanostructure, the maximum phosphate adsorption capacity was 151.98 mg P/g (La), much higher than the result of La(OH)3 nanocrystal, produced by precipitation without PAN or any organic surfactants. Moreover, the PLNFs could remove phosphate (2 mg P/L) to a severe low concentration within 20 min, which could lead to a nutrient deficient condition to protect water quality and ecosystem. The optimization of PLNFs design was implemented through parameter adjustment of electrospinning. Lanthanum salt content, humidity, concentration of solution, applied voltage were chosen to analyze the influences on the composition, diameter and morphology of the nanofibers, giving the result that the most effective adsorbent was the PLNFs with spider-web-like nano-structures.