A pyrimidine derivative and its preparation method and use. The invention discloses a compound represented by formula I or its crystal form and salt: wherein, R1 is selected from hydrogen, halogen, C1-C6 alkyl, C1-C6 alkoxy, amino or substituted amino, and R2-R4 are respectively or At the same time, it is selected from hydrogen, alkali metals, and C1-C6 alkyl groups. The SNNP of compound 1 of the present invention is used as a drug carrier, which significantly improves the therapeutic effect of monomeric nucleoside drugs and / or oligonucleotide drugs, has a good sustained-release effect, enhances the therapeutic effect of drugs, and reduces the dosage of drugs and the frequency of administration, improve the bioavailability of the drug, reduce the toxic and side effects of the drug, and have good clinical application value; at the same time, the preparation method of the present invention has high yield, high purity, low energy consumption, few steps, and easy operation , low cost, environmental protection, safety and other advantages, very suitable for industrial applications.