Find all polynomials $P\in \mathbb{Q}[x]$, which satisfy the following equation:
$P^2 (n)+\frac{1}{4}=P(n^2+\frac{1}{4})$ for $\forall$  $n\in \mathbb{N}$.

To find all polynomials \( P \in \mathbb{Q}[x] \) that satisfy the equation \( P^2(n) + \frac{1}{4} = P\left(n^2 + \frac{1}{4}\right) \) for all \( n \in \mathbb{N} \), we will proceed as follows:

1. **Analyzing the given equation:**
   \[
   P^2(n) + \frac{1}{4} = P\left(n^2 + \frac{1}{4}\right)
   \]
   This equation must hold for all natural numbers \( n \).

2. **Considering the parity of \( P(x) \):**
   Since the equation involves \( P(n) \) and \( P(n^2 + \frac{1}{4}) \), we need to consider whether \( P(x) \) is an odd or even function.

3. **Case 1: \( P(x) \) is an odd function:**
   If \( P(x) \) is odd, then \( P(x) = -P(-x) \). Let \( P(x) = xQ(x^2) \), where \( Q(x) \) is a polynomial. Substituting this into the original equation, we get:
   \[
   (xQ(x^2))^2 + \frac{1}{4} = P\left(x^2 + \frac{1}{4}\right)
   \]
   Simplifying, we have:
   \[
   x^2Q(x^2)^2 + \frac{1}{4} = (x^2 + \frac{1}{4})Q\left((x^2 + \frac{1}{4})^2\right)
   \]
   Since this must hold for all \( x \), we can set \( x = 0 \):
   \[
   \frac{1}{4} = \left(\frac{1}{16}\right)Q\left(\left(\frac{1}{16}\right)^2\right)
   \]
   This implies \( Q\left(\frac{1}{16}\right) = 1 \). By induction, we can show that \( Q(x) = 1 \) for all \( x \), leading to:
   \[
   P(x) = x
   \]

4. **Case 2: \( P(x) \) is an even function:**
   If \( P(x) \) is even, then \( P(x) = P(-x) \). Let \( P(x) = Q(x^2) \), where \( Q(x) \) is a polynomial. Substituting this into the original equation, we get:
   \[
   Q(x^2)^2 + \frac{1}{4} = Q\left((x^2 + \frac{1}{4})^2\right)
   \]
   Simplifying, we have:
   \[
   Q(x)^2 + \frac{1}{4} = Q\left((x + \frac{1}{4})^2\right)
   \]
   Let \( R(x) = Q(x - \frac{1}{4}) \). The equation becomes:
   \[
   R(x)^2 + \frac{1}{4} = R(x^2 + \frac{1}{4})
   \]
   This implies that if \( P(x) \) is an even solution, then there exists another solution \( R(x) \) such that \( P(x) = R(x^2 + \frac{1}{4}) \).

5. **Combining both cases:**
   From both cases, we can see that the solutions can be expressed in terms of compositions of the function \( f(x) = x^2 + \frac{1}{4} \). Specifically, the solutions are:
   \[
   P(x) = f^{[n]}(x)
   \]
   where \( n \in \mathbb{Z}_{\ge 0} \) and \( f^{[n]}(x) \) denotes the \( n \)-th composition of \( f(x) \) with itself, with \( f^{[0]}(x) \equiv x \).

The final answer is \( \boxed{ P(x) = f^{[n]}(x) } \) where \( f(x) = x^2 + \frac{1}{4} \) and \( n \in \mathbb{Z}_{\ge 0} \).