A one-target-multitriggered hybridization chain reaction (MHCR) strategy was designed for ultrasensitive electrochemical detection of DNA by combining the isothermal strand-displacement polymerase reaction (ISDPR) with the DNA self-assembly on a DNA sensor surface. The sensor was constructed by immobilizing a hairpin-like capture probe (CP) on a gold electrode via an Au-S bond. The ISDPR was triggered by the hybridization of the target DNA to open the CP and primer to anneal the complementary part in the bottom of the exposed stem and the extension of the primer in the presence of dNTPs and polymerase. Each target copy could produce a few opened CPs. Afterwards, the other part of the exposed stem acted as an initiator to trigger the hybridization chain reaction (HCR) when incubated with two hairpin monomers. Using Ru(NH3)6 as an electrochemically active indicator to interact with the MHCR product, the amperometric response demonstrated a perfect multiple amplification effect. The constructed sensor showed a high sensitivity for detection of the target DNA in a linear range from 0.1 fM to 10 pM, a detection limit down to 0.02 fM (3σ) and good selectivity for base discrimination. This method did not need any modification or labelling process. The proposed strategy provides a powerful tool for cascade signal amplification and has a wide potential application in bioanalysis.