A compound represented by formula I, or a pharmaceutically acceptable salt thereof, or a tautomer, a solvate, or a polymorph thereof, wherein, Ri is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl and 4-8 membered heteroaryl; Y is O, NH or S; R 2 is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl, 4-8 membered heteroaryl and -C(=O)R 4, wherein R 4 is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl and 4-8 membered heteroaryl; - - represents a double bond or a single bond, when it is a double bond, R 3 is O; when it is a single bond, R 3 is OR 5, For SH, and R 5 is H, Boc, TBS, TES, CH 2 SCH 3, CH 2 OCH 3, - CH 2 OP(=O) 2, -CH 2 OP(=O)(OBn) 2, -OP(=O) 2, -OP(=O)(OBn) 2, -COOH, monosaccharide, folic acid and folic acid analog or monoclonal antibody; each X is independently H, OH or halogen; each of the above term "substituted" independently means that one or more hydrogen atoms on the group are substituted with a substituent selected from the group consisting of: halogen, -OH, NH 2, CN, COOH, -OP(=O) 2, unsubstituted or halogenated C1-C8 alkyl, unsubstituted or halogenated C3-C8 cycloalkyl, unsubstituted or halogenated C1-C8 alkoxy, unsubstituted or halogenated C2-C6 alkenyl, unsubstituted or halogenated C2-C6 alkynyl, unsubstituted or halogenated C2-C6 acyl, unsubstituted or halogenated C2-C6 amido, unsubstituted or halogenated 5-8 membered aryl, unsubstituted or halogenated 5-8 membered heteroaryl, unsubstituted or halogenated 4-8 membered saturated heterocycle or carbocycle; wherein each of the above heteroaryl groups independently contains 1-3 heteroatoms selected from the group consisting of N, O and S. Ri is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl and 4-8 membered heteroaryl; Y is O, NH or S; R 2 is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl, 4-8 membered heteroaryl and -C(=O)R 4, wherein R 4 is a substituted or unsubstituted group selected from the group consisting of: C1-C6 alkyl, C3-C8 cycloalkyl, C2-C6 alkenyl, C3-C8 cycloalkenyl, C2-C6 alkynyl, C6-C10 aryl, C7-C15 arylalkyl and 4-8 membered heteroaryl; - - represents a double bond or a single bond, when it is a double bond, R 3 is O; when it is a single bond, R 3 is OR 5, For SH, and R 5 is H, Boc, TBS, TES, CH 2 SCH 3, CH 2 OCH 3, - CH 2 OP(=O) 2, -CH 2 OP(=O)(OBn) 2, -OP(=O) 2, -OP(=O)(OBn) 2, -COOH, monosaccharide, folic acid and folic acid analog or monoclonal antibody; each X is independently H, OH or halogen; each of the above term "substituted" independently means that one or more hydrogen atoms on the group are substituted with a substituent selected from the group consisting of: halogen, -OH, NH 2, CN, COOH, -OP(=O) 2, unsubstituted or halogenated C1-C8 alkyl, unsubstituted or halogenated C3-C8 cycloalkyl, unsubstituted or halogenated C1-C8 alkoxy, unsubstituted or halogenated C2-C6 alkenyl, unsubstituted or halogenated C2-C6 alkynyl, unsubstituted or halogenated C2-C6 acyl, unsubstituted or halogenated C2-C6 amido, unsubstituted or halogenated 5-8 membered aryl, unsubstituted or halogenated 5-8 membered heteroaryl, unsubstituted or halogenated 4-8 membered saturated heterocycle or carbocycle; wherein each of the above heteroaryl groups independently contains 1-3 heteroatoms selected from the group consisting of N, O and S.