In order to avoid the phase separation of the high density-polyethylene modified asphalt, a composite material modifier was prepared in the melt blending process with High density-Polyethylene (HDPE) and Styrene-butadiene-styrene (SBS) and a filler of Carbon black (CB). The storage stability of composite material (HDPE-SBS) modified asphalt was investigated by hot storage stability test and optical microscopic observation. The storage-stable mechanism of HDPE-SBS modified asphalt was analyzed also. The experimental results indicated that the difference of the softening point after storing at high temperature for 48h was very small. Micrographs of HDPE-SBS modified asphalt demonstrated that HDPE-SBS composite modifier particles dispersed uniformly and compactly and there were no obvious phase separations in the modified asphalt within a certain CB content range.