The invention relates to hydrogel nanoparticles used as an injectable subcutaneous implant agent, which can effectively solves the problem of fulfilling an aim of realizing tumor treatment by locally injecting the hydrogel nanoparticles beside a subcutaneous tumor, changing the hydrogel nanoparticles into a gel and slowly releasing nanoparticles in vivo. The hydrogel nanoparticles are prepared from hydrogel solution and 2-methoxyl estradiol liposome nanoparticle powder or solid liposome nanoparticle powder by a method which comprises the following steps of: adding the 2-methoxyl estradiol liposome nanoparticle powder or the solid liposome nanoparticle powder into the hydrogel solution; and uniformly mixing to form the hydrogel nanoparticles which can be used as an injectable subcutaneous implant agent, wherein 100mg of the 2-methoxyl estradiol liposome nanoparticle powder is added into every 1ml of the hydrogel solution. The injectable hydrogel implant agent of the invention can be used for treating subcutaneous tumors such as breast cancer and the like, keep an effective medicament concentration in a local target site, can realize sustained release and controlled release, can avoid medicament resistance caused by long-term medication, simultaneously improve a curative effect and reduce dosage, reduce toxic and side effects on a whole body, and also can prevent lymphatic metastasis of the tumors such as the breast cancer and the like.