The invention relates to tetrahydroquinoline extract and a preparation method thereof, in particular to a diastereomer mixture of formula (XIII-a) or salt thereof and a preparation method thereof, wherein R<15'> is an optically active protection group of amino, R<2> is hydrogen or optionally substituted alkyl, and R<3> is hydrogen or optionally substituted alkyl; and R<6>, R<7>, R<8> and R<9> are independently hydrogen, halogen, hydroxyl, nitryl, cyano, optionally substituted alkyl, optionally substituted alkoxyl, optionally substituted alkyl mesyloxy or optionally substituted amino; or R<6> and R<7>, or R<7> and R<8>, or R<8> and R<9> can be combined at a tail end to form alkylidene, which comprises 1 to 3 independent hetero atoms selected from nitrogen, sulfur and oxygen atoms and can have a substituted group.