The window period of HIV infection refers to the time between HIV exposure and quantified and consistent detection of viral markers. The seroconversion window period is the interval between HIV infection and the first detection of antibodies. The eclipse period is the initial phase from HIV infection to reliable detection of HIV RNA. Understanding the window period is the basis for HIV test counseling, helping to provide key information about how soon after HIV exposure the tests and repeat tests should be offered and when the HIV infection can be excluded after negative test results are obtained. It has guiding significance for formulating post-exposure testing algorithm, selecting tests and interpreting test results. This paper introduced the definition of window period, emphasized the main points for accurately understanding the concept, analyzed the factors affecting the window period, especially the impact of antiretroviral drugs on viral marker response and detection, and proposed the follow-up method and post-exposure test strategy based on the length of window period, aiming to provide reference for the diagnosis of acute HIV infection.