Field exploratory experimental study was carried out to verify the feasibility of the technology of composite slurry-rockfill structure. The test procedures include slurry preparation, layered rockfill, grouting and mechanical rolling. The test methods include geological radar, drilling and coring, and post-flood inspection. Results reveal that the average porosity of rockfill is 26%-30%, and the fluidity of slurry is 200-240 mm. For rockfills with thicknesses of 0.5 m and 0.75 m, vibrating rolling 6 times and 8 times respectively will meet the requirements of compaction. The radar reflection wave of each grouting roller compacted layer is relatively uniform, and the test result of grouting paste is better than that of grouting mortar. The acquisition rate of core sample is not high, and the compressive strength of core sample is 8.5-15.1 MPa. Inspection after the flood season shows that the structure of the test area is complete, stable and undamaged. To improve the construction efficiency and grouting effect, we recommended removing the particles below 5 mm and humidifying in advance and adjust the slurry formula according to the on-site environment. Equipment such as tandem cylinders, chutes, or method of spraying grouting at multiple points can also be employed. The technical idea of composite grout rockfill structure helps to increase the compressive deformation modulus of the rockfill foundation and reduce the overall settlement of the ultra-high rockfill dam, thus providing technical support for the construction of 300 m level ultra-high rockfill dam.