The invention relates to an in-situ detection device for the 
chlorophyll concentration of water and a detection method of the in-situ detection device. The in-situ detection device and the detection method aim at solving the problems that in an existing device, the excitation light path can be not adjusted, the 
fluorescence intensity is low, and the 
fluorescence detection accuracy and sensitivity are low due to scattering generated when water contains lots of impurities. A 
water sample chamber water inlet device is arranged at the bottom of a 
quartz water sample chamber, a stainless steel shell is arranged outside the 
quartz water sample chamber, a 
detector, a 
light filter and a focusing lens are arranged over the 
quartz water 
sample chamber, a 
light source, a 
light source transverse adjustment support, a 
light source transverse adjustment mechanism and a light source direction adjustment mechanism are arranged on the side face of the quartz water 
sample chamber, and the 
detector, the light source, the light source transverse adjustment mechanism and a light source incidence angle adjustment mechanism are connected with a host through wires. The method includes the steps that water 
background fluorescence signals measured under the off-state of the excitation light source are collected, then water 
fluorescence signals under the on-state of the excitation light source are collected, and 
background fluorescence interference is automatically subtracted through operation.