ag.algebraic geometry - Is line bundle determined by the parameter space and fiber?

Yes, you can. This follows from the see-saw principle for instance. You can also argue directly as follows. The spaces of global sections $H^{0}(Y,f_*L)$ and $H^{0}(X,L)$ are naturally isomorphic. Since $f_*L$ is a trivial line bundle we can choose a global nowhere vanishing section $e$ of $f_*L$. Let $s in H^{0}(X,L)$ be the section of $L$ corresponding to $e$ under the above isomorphism. To show that $L$ is trivial it suffices to check that $s$ does not vanish anywhere. But if $x in X$ is a closed point where $s$ vanishes, then if we restrict $s$ to the fiber $X_{f(x)}$, we will get a section of the structure sheaf of integral projective variety which vanishes at a point. Thus the restriction of $s$ to $X_{f(x)}$ must be identically zero. This shows that $e$ vanishes at $f(x)$ which is a contradiction.

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