A system for calculating a droplet delay time in a liquid flow of a sorting device, comprising: a first laser source configured to emit a first laser beam to the liquid flow of the sorting device, the first laser beam and the liquid flow intersecting at a first laser interrogation point located in a nozzle system of the sorting device; a second laser source configured to emit a second laser beam to the liquid flow, the second laser beam and the liquid flow intersecting at a second laser interrogation point located outside the nozzle system, wherein the second laser interrogation point is before a droplet separation point; a first detector corresponding to the first laser source and configured to detect emission, in response to the first laser beam, of a particle in the liquid flow to determine the time when the particle passes through the first laser interrogation point; a second detector corresponding to the second laser source and configured to detect emission, in response to the second laser beam, of the particle in the liquid flow to determine the time when the particle passes through the second laser interrogation point; a camera device adapted to obtain an image of the liquid flow, the secon laser interrogation point and the droplet separation point being within a pick-up range of the camera device; a droplet delay time calculation unit configured to calculate, on the basis of the time when the particle in the liquid flow passes through the first laser interrogation point and the time when the particle passes through the second laser interrogation point, a first delay time for the liquid flow flowing from the first laser interrogation point to the second laser interrogation point, and calculate, on the basis of the first delay time, a droplet delay time; wherein the system is characterized in that when the system is in use, the second laser interrogation point is arranged before a droplet separation point, and the droplet delay time calculation unit is further configured to calculate a second delay time for the liquid flow flowing from the second laser interrogation point to the droplet separation point and to calculate the second delay time by using a plurality of images fo the liquid flow obtained by the camera device, and to calculate the sum of the first delay time and the second delay time as the droplet delay time. a first laser source configured to emit a first laser beam to the liquid flow of the sorting device, the first laser beam and the liquid flow intersecting at a first laser interrogation point located in a nozzle system of the sorting device; a second laser source configured to emit a second laser beam to the liquid flow, the second laser beam and the liquid flow intersecting at a second laser interrogation point located outside the nozzle system, wherein the second laser interrogation point is before a droplet separation point; a first detector corresponding to the first laser source and configured to detect emission, in response to the first laser beam, of a particle in the liquid flow to determine the time when the particle passes through the first laser interrogation point; a second detector corresponding to the second laser source and configured to detect emission, in response to the second laser beam, of the particle in the liquid flow to determine the time when the particle passes through the second laser interrogation point; a camera device adapted to obtain an image of the liquid flow, the secon laser interrogation point and the droplet separation point being within a pick-up range of the camera device; a droplet delay time calculation unit configured to calculate, on the basis of the time when the particle in the liquid flow passes through the first laser interrogation point and the time when the particle passes through the second laser interrogation point, a first delay time for the liquid flow flowing from the first laser interrogation point to the second laser interrogation point, and calculate, on the basis of the first delay time, a droplet delay time; wherein the system is characterized in that when the system is in use, the second laser interrogation point is arranged before a droplet separation point, and the droplet delay time calculation unit is further configured to calculate a second delay time for the liquid flow flowing from the second laser interrogation point to the droplet separation point and to calculate the second delay time by using a plurality of images fo the liquid flow obtained by the camera device, and to calculate the sum of the first delay time and the second delay time as the droplet delay time.