The in-situ construction and performance of a MEMS（micro-electro-mechanical system） planar micro detonator were introduced. The micro detonator was composed of MEMS Ni/Cr heater and direct-writing primary charge， constructed on the same plane of a silicon nitride wafer. The primary explosive cavity was etched on one side of the Ni/Cr heater， and the bridge of the heater is constructed inside the cavity. The porous copper precursor ink was written into the cavity and was converted into copper azide primary explosive by in-situ azidation reaction and the MEMS detonator was obtained. The average resistance of the micro detonator is about 4 Ω， with the initiation delay time of 8.44 μs， the 50% initiation voltage of 14.29 V， and the initiation energy of 0.33 mJ. The average charge of the detonator is 5.18 mg with a mass RSD（relative standard deviation） of 2.6%. The detonator can detonate the hxanitrohexaazaisowurtzitane （CL-20） secondary explosive reliably and completely.