One of the commonly used test method for roadbed stiffness is compaction degree examination. The accuracy of indoor standard compaction test result is paramount, which can be influenced by water content, soil type, plasticity index, wet method, dry method, soil sample reusing and residual soil height. The author expatiates those influence factors presented above with data in the paper. Furthermore, further researches are made on test data processing, phenomenon that compaction degree is always more than 1 during the actual construction and the water content scope of compacted roadbed. Based on the analysis of the factors and problems presented above, rational test method was proposed to guide the field construction.