A method for detecting nano-particles, comprising the following steps: compressing a sample liquid to be detected into a sample liquid flow by hydrodynamic focusing using a sheath fluid, wherein the sample liquid flow contains nano-particles to be detected, and the nano-particles are essentially separated from each other and essentially flow in the same line in the sample liquid flow, wherein the nano-particles are at least one of prokaryotic cells, organelles, viruses, polystyrene nano-particles and silicon dioxide nano-particles; irradiating a measuring light to the sample liquid flow, wherein a single nano-particle in the sample liquid flow is irradiated by the measuring light for 0.2-2 ms; defining the region of the sample liquid flow subjected to irradiation of the measuring light as a detecting region, and collecting light signals emitted from the region irradiated by the measuring light via a lens imaging system, and allowing the light signals collected by the lens imaging system to pass through a field stop, so as to filter out the light signals outside the detecting region and enrich the light signals from the detecting region, wherein the volume of the detecting region is 1-100 fL, preferably 10-100 fL, more preferably 10-30 fL; subjecting the light signals enriched by the field stop to optoelectronic signal conversion, and acquiring the electrical signals of scattered light emitted from each nano-particle in the sample liquid flow, or the electrical signals of scattered light and fluorescent light emitted from each nano-particle in the sample liquid flow, the field stop is a photosensitive region of the photoelectric detector or an aperture diaphragm added in front of the photoelectric detector, the area of the photosensitive region or the aperture area of the aperture diaphragm is 50 to 5×10 6 μm 2, preferably 2×10 3 to 2×10 5 μm 2, more preferably 8×10 3 to 8×10 4 μm 2; carrying out qualitative and/or quantitative analysis for the nano-particles according to the electrical signals. compressing a sample liquid to be detected into a sample liquid flow by hydrodynamic focusing using a sheath fluid, wherein the sample liquid flow contains nano-particles to be detected, and the nano-particles are essentially separated from each other and essentially flow in the same line in the sample liquid flow, wherein the nano-particles are at least one of prokaryotic cells, organelles, viruses, polystyrene nano-particles and silicon dioxide nano-particles; irradiating a measuring light to the sample liquid flow, wherein a single nano-particle in the sample liquid flow is irradiated by the measuring light for 0.2-2 ms; defining the region of the sample liquid flow subjected to irradiation of the measuring light as a detecting region, and collecting light signals emitted from the region irradiated by the measuring light via a lens imaging system, and allowing the light signals collected by the lens imaging system to pass through a field stop, so as to filter out the light signals outside the detecting region and enrich the light signals from the detecting region, wherein the volume of the detecting region is 1-100 fL, preferably 10-100 fL, more preferably 10-30 fL; subjecting the light signals enriched by the field stop to optoelectronic signal conversion, and acquiring the electrical signals of scattered light emitted from each nano-particle in the sample liquid flow, or the electrical signals of scattered light and fluorescent light emitted from each nano-particle in the sample liquid flow, the field stop is a photosensitive region of the photoelectric detector or an aperture diaphragm added in front of the photoelectric detector, the area of the photosensitive region or the aperture area of the aperture diaphragm is 50 to 5×10 6 μm 2, preferably 2×10 3 to 2×10 5 μm 2, more preferably 8×10 3 to 8×10 4 μm 2; carrying out qualitative and/or quantitative analysis for the nano-particles according to the electrical signals.