The invention relates to a water soluble chitosan derivative and the preparation method and application of the water soluble chitosan derivative. The water soluble chitosan derivative adopts the chitosan as raw material to synthesize substitutional derivatives on the chitosan -NH2 with the following structural formula (I). The preparation method is making the chitosan react with an ester with a methacrylic oxyl on one end and a propylene oxyl on the other end in the medium of water, acetic acid and ethanol, obtaining the chitosan derivative with both polymerized unsaturated carbon-carbon double bond and water solubility at the same time. The the preparation method for the water soluble chitosan derivative has the advantages of mild conditions, simple operation, natural and non toxic prepared chitosan derivatives, biodegradation and biocompatibility, and ability to be used as injectable hydrogel support and cartilage repair materials for tissue engineering scaffold material after further polymerization.