The leukocyte enzyme myeloperoxidase (MPO) is widely present in animals, and is involved in various physiological responses in vivo. When the body is stimulated and the stress reaction is produced, MPO content increases rapidly, showing strong oxidation activity and destruction of invading microorganisms, and enhancing immunity. The strong oxidizing property of MPO has an immense influence on the oxidative metabolism of drugs. Praziquantel (PZQ) is a quinoline pyrazine derivative, and widely used in aquaculture as an anti-parasite drug. In this experiment, PZQ was used as a model drug and crucian carp as a model organism. Analysis of the relationship between MPO mRNA expression and concentration of PZQ in the crucian carp was made after single-dose oral administration of PZQ in an attempt to establish a method for the evaluation of fish in vivo drug residues with MPO genetic. In this study, blood samples were collected at 0.25 h, 0.5 h, 1 h, 3 h, 6 h, 12 h, 24 h, 48 h and 96 h after oral administration of 10 mg/kg PZQ, and PZQ concentrations of the samples were simultaneously detected using high-performance liquid chromatography. Pharmacokinetic analysis was performed using the DAS 3.0 program. Pharmacokinetic parameters were based on classical compartmental model analysis. PZQ can be absorbed quickly into the blood and eliminated rapidly. Concentration and time data of PZQ in the blood can best be described by a two-compartmental open pharmacokinetic model; the pharmacokinetics equation was t-21.409e-^{1.206. The peak concentration of PZQ was 2.85 μg/mL in the first hour and it was below the detection limit after 96 h. The relative expression of MPO was explored for the first time, using real-time fluorescent quantitative PCR (RT-PCR) with the reference gene of -actin. The peak value in the relative expression of MPO mRNA also appeared in the first hour. At 0.25, 0.5, 1, 3 and 6 h the relative expression of MPO mRNA showed an extremely significant difference compared with the control group (<0.05), then gradually decreased to a relatively low value (no significant difference) with the control group at 48 h. In addition, the difference between the control group and the blank control group was 0.993, which showed that the full price feed and the mouth irrigation method had no effect on the experimental results. Correlation analysis indicated that the correlation coefficient of the MPO mRNA relative expression amount and PZQ concentration in blood was highly correlated (=0.96). This suggests that MPO may participate in the oxidative metabolism of PZQ. This study provides a new approach to the molecular evaluation of drug residues in aquatic animals, and a theoretical basis for the safe use of drugs in aquaculture.}