In this paper, a set of pyroclastic rocks was discovered in the original Heilongjiang Group of Yilan area. They are composed of debris and fine-grained pyroclastics. The debris is in the lenticular and flame forms, whereas the fine-grained pyroclastics have been highly deformed. With similar composition, they are comprised of sodium amphibole, albite, epidote and minor amounts of chlorite; evertheless, there is an obvious difference between them in mineral content and mineral characteristics. Geochemical characteristics of pyroclastic rocks suggest that the debris belongs to alkaline basalt, with its LREE/HREE ratio between 3.87 and 4.24, its La/Yb)n between 7.20 and 8.12 , its Eu */Eu ratio between 1.03 and 1.09 and its REE distribution partition curve being of the right-oblique type. Compared to things of MORB, the trace elements of pyroclastics are more enriched in such incompatible elements as Sr, K, Rb and Ba. Yb is slightly lower than that in MORB. In the Zr-Zr/Y diagram and TiO-MnO×10[CD-P2O5×10 diagram, points of all samples are plotted in the oceanic-island basalt and interpolated area. It is considered that pyroclastics are alkaline basalts formed in an ocean island environment within the oceanic plate. Before the complete coagulation of these oceanic-island alkaline the hotspot in the oceanic plate became active again and broke the previously-formed alkaline basalts intoplastic-semiplastic debris, which was then cemented by pyroclastics. The pyroclastics are ocean island type pepyroclastics formed by “off-axis volcanism" within the oceanic plate. This discovery has provided some new clues to the paleotectonic environment and to the determination of tectonic attributes of the original Heilongjiang Group