Abstract: Through Analyzing the phenylalanine ammonialyase(PAL) activity variety regularity of the leaves after sprayed Harpins protein with different density on indoor cucumber, the thesis probed the mechanism of preventing and curing cucumber with Harpins protein,the result showed：the PAL activity variety during different periods after spraying Harpins protein is different, the PAL activity variety regularity in the leaves has no significant difference, comparing with the check experiment after inoculating Harpin protein with 0.005g/L; however, when inoculating Harpin protein with 0.015g/L,0.045g/L, that of difference is significant , comparing with the check experiment , The PAL activity variety after inoculating Harpins protein can be used as a physiological and chemical index sign, which can be used as a testimony of protecting and curing cucumber frost mildewed disease with Harpins protein.