[Objective] The purpose of this study is to isolate and identify antagonistic myxobacteria against Phytophthora infestans from soil samples collected from a farmland near the Yellow River and analyze the stability of the fermentation supernatant of the antagonistic strain and the inhibition effect on the pathogen of potato late blight preliminarily. These findings may lay a foundation for the isolation and identification of the antibiotic substances and the development of new biological pesticides resistant to potato late blight. [Methods] The strain was isolated by the rabbit dung baiting method. The antagonistic strains were screened by plate confrontation assay and identified by its morphological, physiological, and biochemical characteristics, and its 16S rRNA gene sequence. The growth curve of strain YR-7 was tested by a weighing method. Inhibitory rate of the fermented supernatant at different stages on the mycelial growth of P. infestans and stability of the fermented supernatant was tested by a plating method. Effect of the fermented supernatant of strain YR-7 on disease prevention for detached potato leaves was evaluated by spraying concentrated fermentation supernatant on the detached potato leaves. [Results] Seven strains of myxobacteria were isolated and four of them presented antagonistic activity against P. infestans. The strain YR-7 with the strongest antagonistic activity showed an inhibition rate of 96% to P. infestans and was identified as Myxococcus xanthus. After the strain YR-7 was cultured for 7 days, inhibitory effect of its fermented supernatant on the mycelial growth of P. infestans tended to be stable. Its activity could be maintained from 30 °C to 50 °C (inhibition rate of 50.90%) and decreased when the temperature exceeded 50 °C. Its activity still remained an inhibition rate of 25.45% after treated at 90 °C for 1 h. The concentrated fermentation supernatant was stable from pH 4.0 to 9.0 and its inhibition rate was more than 40.21%. Its activity significantly decreased when the pH value was less than 4.0 or greater than 9.0. The active substances could not be degraded by protease and their activity was not affected by UV and natural light exposure. The relative lesion area of potato leaves declined from 68.19% to 0.35% when the concentrated fermentation supernatant was sprayed on the leaves before the spore suspension of P. infestans was inoculated. [Conclusion] The strain YR-7 can produce secondary metabolites against the pathogen of potato late blight. The antibiotic substances have good stability and could effectively inhibit P. infestans from infecting potato leaves, which have the potential value for developing biological pesticides resistant to potato late blight.