Abstract: Provided is a magnetic signal detection chip. Each flow sensing component thereof includes lead portions and notch portion(s) located between the adjacent lead portions, wherein the notch portion(s) is(are) located in the flow path and at least a part of each lead portion adjacent to the notch portion is exposed to the chemical fluid, so that the flow sensing component(s) is(are) conducted when the chemical fluid completely covers the notch portion(s). Thus one could regulate the processing steps according to the sensing results of the flow sensing component, which can avoid inaccurate detection results. Further provided is a detection card including the magnetic signal detection chip and further provided is a nucleic acid detection device including the detection card. Further provided is a method of detecting composition containing target DNA fragment in chemical fluid.