Abstract: In order to efficiently detect the colonization of Phytophthora parasitica var. nicotianae in tobacco planting soil, a quantitative PCR method was researched based on locked nucleic acid (LNA) primer. The results showed that: 1) Comparing with conventional DNA primers, the LNA primer raised PCR reaction annealing temperature, reduced the formation of primer dimers and non-specific products, and improved the sensitivity and specificity of detection. 2)LNA quantitative PCR analysis revealed that the concentration of Phytophthora parasitica var. nicotianae in 14 soil samples of tobacco / garlic rotation fields ranged from 2.76×10³ to 5.20×10⁴ per gram. This method is sensitive and efficient, the detection can be completed within 4-5 hours.