Extramedullary infiltration (EMI), as a concomitant symptom of acute myeloid leukemia (AML), is associated with low complete remission and poor prognosis in AML. However, the mechanism of EMI remains indistinct. Clinical trials showed that increased miR-29s were associated with a poor overall survival in AML [14]. Nevertheless, they were proved to work as tumor suppressor genes by encouraging apoptosis and inhibiting proliferation in vitro. These contradictory results led us to the hypothesis that miR-29s may play a notable role in the prognosis of AML rather than leukemogenesis. Thus, we explored the specimens of AML patients and addressed this issue into miR-29c&b2 knockout mice. As a result, a poor overall survival and invasive blast cells were observed in high miR-29c&b2-expression patients, and the wildtype mice presented a shorter survival with heavier leukemia infiltration in extramedullary organs. Subsequently, we found that the miR-29c&b2 inside leukemia cells promoted EMI, but not the one in the microenvironment. The analysis of signal pathway revealed that miR-29c&b2 could target HMG-box transcription factor 1 (Hbp1) directly, then reduced Hbp1 bound to the promoter of non-muscle myosin IIB (Myh10) as a transcript inhibitor. Thus, increased Myh10 encouraged the migration of leukemia cells. Accordingly, AML patients with EMI were confirmed to have high miR-29c&b2 and MYH10 with low HBP1. Therefore, we identify that miR-29c&b2 contribute to the poor prognosis of AML patients by promoting EMI, and related genes analyses are prospectively feasible in assessment of AML outcome.