How to reduce measuring angle of JSN SR04T?
This question probably needs to be in another section, like physics. But be that as it may, there are practical ways to control the field of view (FOV):


Use a phased array of several transducers and mix their delayed signals such that the timing is for ultrasonic waves from a certain direction. Also, if the speed of sound varies, you'll need to adjust those delays accordingly.
If the speed of sound is fairly constant, and you want to use only one transducer, then you can place the transducer in a box with walls that absorb the sound, but with a window containing an FZP lens for 40kHz. And, of course, the transducer would need to be at the focal point of that lens, and pointing toward the lens. (Note that although the spec sheet doesn't say, ultrasonic transducers are typically very narrow-band, which is good because FZP lenses have very narrow bandwidth).
Place the transducer pointing in to one end of tube that is at least several wavelengths long, but with many small holes drilled along its length such that the total hole area is comparable to the area of the opening at the far end. (There should be at least several holes per wavelength along the tube, and don't put the holes all on the same side of the tube).
Use a megaphone.