The construction of isothermal nucleic acid amplification systems with extremely high signal amplification abilities is of great importance for biomarker detection and disease diagnosis. Herein, for the first time, we proposed an intermolecular and intramolecular priming co-directed synergistic multi-strand displacement amplification (SM-SDA) system for microRNA (miRNA) detection. Strategically, the SM-SDA system is made of a single multi-functionalized hairpin probe (MF-HP) that is engineered with a long stem and endowed with target complementation, configuration transformation, enzyme recognition, and signal reporting abilities. The presence of a specific target, the model of miRNA-21, to react with MF-HP would cause an intermolecular priming-directed strand replication in parallel with an intramolecular priming-directed strand replication. The co-directed priming pattern results in the occurrence of SM-SDA containing a target-induced cyclical strand displacement amplification (T-CSDA), a target analogue primer (TAP)-induced CSDA (TAP-CSDA), and a nicked trigger primer (NTP)-induced CSDA (NTP-CSDA). The resulting multiple circuits with a synergetic signal amplification capacity remarkably enhance the target miRNA response, which has not only improved the sensitivity for qualitative and quantitative detection of miRNA but also realized the analysis of target miRNA from real clinical samples. As a proof-of-concept study, this simple but attractive isothermal signal amplification system holds a great potential for molecular diagnosis of diseases and will stimulate interest, new ideas, and discoveries in this fascinating field.