Black spot disease caused by Alternaria alternata is one of the main diseases of fruits and vegetables after harvest. The aims of this study were to screen lactic acid bacteria (LAB) with antagonistic activity to A. alternata， and to preliminarily explore the antibacterial mechanism of LAB. The strains with strong inhibitory activity against A. alternata was screened by fungus cake method， and it was identified by biochemical tests and 16S rDNA sequencing. The inhibition mechanism of L. plantarum was preliminarily investigated by the determination of cell membrane integrity， malondialdehyde， soluble protein， soluble sugar content， and spore ultrastructural changes. Results indicated that the strain LNL2-4-2 had a strong inhibitory effect on A. alternata， and the inhibition rate was 80.86%. It was identified as Lactobacillus plantarum. Different concentrations of strain LNL2-4-2 inhibited the growth of A. alternata mycelium and spore germination， and the inhibition effect increased with increasing concentration. When the concentration of strain LNL2-4-2 was 109 CFU/mL， the membrane integrity rate of A. alternata spores decreased by 10.98%， and the extracellular malondialdehyde， soluble protein and soluble sugar content increased by 10.66%， 3.10% and 11.21%， respectively. The scanning electron microscope images showed that strain LNL2-4-2 damaged A. alternata plasma membrane， resulting in spore surface to shrink， the edges to be blurred， and the top to disrupt. In the study of apple biological control， when stored for 6 days， the inhibition rate of LNL2-4-2 at a concentration of 103 CFU/mL on the diameter of the damage caused by A. alternata reached 47.24%. These results indicated that the action site of L. plantarum LNL2-4-2 on A. alternata was cell membrane of spores， which exerted antagonism through membrane damage. The study proved that L. plantarum LNL2-4-2 can act as a biological preservative for inhibiting A. alternata.