I would like to perform glare analysis on the basis of HDRI, my fisheye lens is Canon EF 8-15mm f/4L fisheye USM lens. Theoretically, the view angle of the fisheye lens is 180 degree, however, the real view angle has always a small deviation. Thus are there any simple methods to measure the real view angle of a fisheye lens?

I just read your paper,and still have something not clear.For example,
1.What is the distance between both grid using to determine the real view angle or can be any distance?
2. What is the grid spacing?

These two distances are related, you need to calculate them.
You first define the distance between both grids as is practical for you (e.g. it can be the distance between the walls of the room where you plan to do the measurements).
Then, you apply the SOHCAHTOA trigonometric formula (I think in your case, it is the TOA: tangent=opposite/adjacent) to calculate what is the grid spacing corresponding to 1°, 2°, 3°, etc.

For instance, tan(1°) * (distance between camera no-parallax point and grid) = (grid spacing)