Abstract: Combining with the fill dam engineering using fly ash in Taiyuan,a series of tests have been carried out for fly ash in roller fill dam using a por table dynamic penetrometer with variable energy.First,the contrast analysis had been done,which compared to other routine methods of in-situ test,including st at ic cone penetration test,dynamic penetration test and standard penetration test, etc.Second, coupled with the apparatus,the compaction tests were also performed on fly ash under the same conditions.Then,on the basis of the results obtained, the geotechnical properties of fly ash,especially its compactness are analyzed.Comp ared with other routine methods measuring the mechanical index of fly ash,t he indirect and simple method was proposed to evaluate the compactness of fly as h by mechanical resistance measured in-situ with the penetrometer.