The invention provides an SOI horizontal 
power MOSFET device and belongs to the technical field of power 
semiconductor devices. A 
dielectric groove is introduced in a drift region and filled with two or more 
dielectric materials, and the 
dielectric coefficients of the dielectric materials are lower than the dielectric coefficient of an 
active layer and are gradually decreased from bottom to top; the side, close to a 
body region, of the dielectric groove is of a 
body region longitudinal extension structure; a 
semiconductor buried layer opposite to the drift region in 
doping type is arranged between the dielectric groove and a dielectric buried layer. Because the dielectric groove filled with the variable k dielectric materials has an modulating action on an internal 
electric field of the 
active layer and plays a role in longitudinally folding the drift region, the 
voltage resistance of the device is greatly improved, and the horizontal size of the device is reduced; because of introduction of the 
body region longitudinal extension structure and the 
semiconductor buried layer, the 
voltage resistance of the device is further improved, the exhausting action on the drift region is enhanced, the 
doping concentration of the drift region can be improved, and therefore the 
on resistance of the device is reduced; the grid and drain 
capacitance of the device can be further reduced through the dielectric groove, and the frequency and output power of the device are improved.