Measuring sound absorption in-situ using pressure and particle velocity sensors has the advantages of restricting background noise and characterizing materials in their target environments in a broadband frequency range. However, deviations exist due to various factors in the in-situ measurements. On the one hand, certain assumptions for the calculation models may not be met in real measurement scenarios, which is a problem that the standardized reverberation room method also has. On the other hand, the operation procedures to perform the measurements and the backplate to support the material under test can also bias the results. In this work, different materials with several experimental factors in the in-situ environments are measured, and the errors of the sound absorption are investigated.