The influences of the applied bias voltage and the pre-irradiation treatment by alpha particles on the electrical properties of sandwich structural diamond film detectors under 5.5 MeV 241Am alpha-particle irradiation were investigated. Results of current- voltage(I-V) and pulse height distribution measurements showed that the dark current of the diamond film detector would increase due to the pre-irradiation by alpha particles. Under the alpha irradiation, the detector under negative bias voltage had a higher response current and a better signal-to-noise ratio than that under a positive bias. Raman scattering studies directly demonstrated that the above phenomenon resulted mainly from the different structural imperfection distributions along the thickness direction. An energy resolution of about 25.0% was obtained for the detector under a negative bias voltage and 38.4% under a positive bias voltage. With increasing alpha-particle irradiation time, both the response current and the charge collection efficiency increased obviously.