The invention relates to a testing reagent for activated partial thromboplastin time. The testing reagent is prepared from phospholipids, activator and stabilizer. The stabilizer is prepared from, by mass, 1%-4% of alanine and 0.5-5 mM of catechin. A preparation method of the testing reagent includes the steps that an HEPES buffer solution is prepared, the pH value of the buffer solution is adjusted to be neutral, and sodium chloride is added; the activator is added to the buffer solution, phospholipids and other components are added after stirring and even mixing are conducted, and then the testing reagent for APTT is obtained after even mixing, stirring and filtering are conducted. According to the testing reagent for activated partial thromboplastin time, stabilizer components with high toxicity are removed, so that production and use processes are safer; meanwhile, the stability and detection accuracy of the testing reagent for APTT are greatly improved.