The invention relates to an on-line rapid detection system for density of an emulsion explosive semi-finished product, which comprises an automatic sampling module, a serial sensor module, a data processing and calculating module, a center control module and a display control module. The system is characterized in that a feeding solenoid valve in the automatic sampling module is switched on, the automatic on-line sampling is realized through a sample outlet and a sample transmission pipe; the sample enters into the serial sensor module through the sample transmission pipe, the sample quality and volume signals can be obtained by two weighing machines and transmitters, and are transmited to the data processing and calculating module; the density detection value is obtained by a weighing signal transition unit, a sample amount determine unit, a density calculating unit and a density correction unit, when the density detection value is provided in the preset value scope, the system enables normal operation; when the density value is greater than the preset value scope, the alarm is carried out, and the feeding speed of a density conditioning agent is changed, the density of the emulsion explosive semi-finished product is adjusted; when the density value is less than the preset value scope, the alarm is carried out and the system is stopped, then the operation is continued after the technological parameter is changed. The on-line rapid detection system can realize the integrated function of automatic sampling, rapid detection and machine stopping by alarming.