The present disclosure provides a microfluidic device and a manufacturing method therefor, and a method for detecting the number of biomolecules and a system for detecting the number of biomolecules. The microfluidic device includes a glass substrate; a plurality of first metal strips formed on the glass substrate; a photoresist covering the plurality of first metal strips; a fluid cavity formed in the photoresist; a plurality of second metal strips formed on the photoresist, wherein a head end of one first metal strip and a tail end of the other first metal strip in any adjacent two first metal strips are electrically coupled through a head end and a tail end of one second metal strip between the two first metal strips, so that the plurality of first metal strips and the plurality of second metal strips form a hollow inductor structure around the fluid cavity.