Let $A=k[x_1,ldots,x_m]$ and $B=k[y_1,ldots,y_n]$ over some algebraically closed field $k$. Let $psi : mathbb{A}^nrightarrow mathbb{A}^m$ be the morphism corresponding to a $k$-algebra homomorphism $phi: Arightarrow B$. Let $a=(a_1,ldots,a_n)in mathbb{A}^n$ and $psi(a)=(b_1,ldots,b_m)inmathbb{A}^m$. I want to show that $phi^{-1}((y_1-a_1,ldots,y_n-a_n))=(x_1-b_1,ldots,x_m-b_m)$.

I know that since $phi$ is a $k$-algebra homomorphism and $A$ and $B$ are finitely generated $k$-algebra, then the preimage of a maximal ideal is again a maximal ideal. I also know that by Hilberts Nullstellensatz, then a point $(a_1,ldots,a_n)$ in a affine variety corresponds to the maximal ideal $(a_1-p_1,ldots,x_n-a_n)$. And by how $psi$ is defined on the point $a$, then this seems to be natural that $phi^{-1}((y_1-a_1,ldots,y_n-a_n))=(x_1-b_1,ldots,x_m-b_m)$. However I was wondering if there were a more rigorously way of doing it or seeing it. My argument seems a little loose.

Your argument is the right idea. Let me try to give you some help.

A $k$-point in $Bbb A^n$ is the same as a map from $Bbb A^0to Bbb A^n$ picking out the point we care about. On the coordinate algebra side, this is represented by the map $k[y_1,cdots,y_n]to k$ given by $y_imapsto a_i$, and the preimage of the maximal ideal $(0)subset k$ is exactly the maximal ideal $(y_1-a_1,cdots,y_n-a_n)$.

Composing this with our map $Bbb A^nto Bbb A^m$, we get a map $Bbb A^0to Bbb A^m$ which corresponds to $k[x_1,cdots,x_m]to k$. The map $Bbb A^0to Bbb A^m$ picks out $(b_1,cdots,b_m)$, so we have that our map on coordinate rings $k[x_1,cdots,x_m]to k$ is given by $x_jmapsto b_j$, and the preimage of the maximal ideal $(0)subset k$ is exactly the maximal $(x_1-b_1,cdots,x_m-b_m)$.

Now look at the sequence of maps $k[x_1,cdots,x_m]to k[y_1,cdots,y_n]to k$. The preimage of $(0)$ along the first map is $(y_1-a_1,cdots,y_n-a_n)$, and the preimage of $(0)$ along the composite is $(x_1-b_1,cdots,x_m-b_m)$. But this means that the preimage of $(y_1-a_1,cdots,y_n-a_n)$ is $(x_1-b_1,cdots,x_m-b_m)$ by, for instance, the second-to-last line of this answer.