Sum of the two or more non-standard uniform distribution? Can anyone show the sum of two $U(-1,1)$?
Any statistical methods to solve the sum of three and more $U(-1,1)$?
I am doing a uniform random walk simulation. Can simulate using coding not can not compare with the theoretical values?

To quickly answer this: The PDF of the sum of $n$ iid $U(-1,1)$ distributed random variables is (for a derivation see here):
$$
(1/2)f_X\left(\frac{x + n}{2};n\right)
$$
where $f_X$ denotes the PDF of the standard Irwin-Hall distribution which is:
$$
f_X(x;n)=\frac{1}{2(n-1)!}\sum_{k=0}^{n}(-1)^k {n\choose k}(x - k)^{n-1}\operatorname{sign}(x - k)
$$
where $\operatorname{sign}$ is the sign function. So for example for $n=2$ the PDF is:
$$(1/2)f_X\left(\frac{x + 2}{2};2\right)=
\begin{cases} 
      \frac{2 + x}{4}& -2\leq x < 0\\
      \frac{2 - x}{4} & 0\leq x\leq 2 \\
      0 & \text{else} 
   \end{cases}
$$
