The invention provides a microfluidic device for automatic extraction of nucleic acid. The microfluidic device comprises a microfluidic chip and a driving device. Before an experiment, a sample, a lysis solution, magnetic beads and the like are injected into a lysis chamber of the microfluidic chip respectively by virtue of an injection hole; a cleaning solution is injected into three washing chambers; an eluent is injected into an elution chamber; then the microfluidic chip is put into a chip tray, and tight fit is formed; a step motor drives the rotation of the chip tray and the positioning control of a groove type optocoupler; the adsorption effect of upper and lower magnets on the magnetic beads is utilized to ensure that the microfluidic chip rotates back and forth in a small range to promote the reaction between the sample and the lysis solution, between the magnetic beads and the cleaning solution and between the cleaning solution and the eluent, and by virtue of a baffle plate at the inlet of each reaction chamber, the transfer between the magnetic beads and to-be-extracted nucleic acid can be performed among the chambers, thereby finishing multiple reaction steps of cell lysis, nucleic acid purification and nucleic acid elution to obtain a purified nucleic acid template and realize automatic extraction of nucleic acid.