Drilling in gravel stratum is one of present difficulties in drilling area with low drilling speed, short service life of bit and high drilling cost. Experimental study was made on diamond bit with new structures in order to improve gravel stratum drilling and developed a diamond-polycrystalline compound bit. Field tests indicate that this type of diamond bit has a promising application prospect based upon the fact that the drilling rate of diamond bit can be no less than 1.8m/h and the service life of bit reach to 50m with the lower drilling cost in addition.