The invention belongs to the nanotechnical field, and discloses preparation of a methoxy polyethylene glycol-poly(lactic-co-glycolic acid)-polylysine(mPEG-PLGA-PLL) cationic polymer nano-drug delivery system and application thereof. The nano-drug delivery system can have multi-functional characteristics such as tumor targeting, reversing drug resistance and medical diagnosis functions through modification, and can be used for supporting organic medicaments, water-soluble medicaments, non-water-soluble medicaments, or developers for diagnosis. The preparation method is simple and convenient, is suitable for mass production, and is particularly suitable for the preparation of targeting drug delivery systems.