We demonstrate an ultrastable miniaturized transportable laser system at 1550 nm by locking it to an optical fiber delay line (FDL). To achieve optimized long-term frequency stability, the FDL was placed into a vacuum chamber with a five-layer thermal shield, and a delicate two-stage active temperature stabilization, an optical power stabilization, and an RF power stabilization were applied in the system. A fractional frequency stability of better than $3.2\times {10}^{-15}$ at 1 s averaging time and $1.1\times {10}^{-14}$ at 1000 s averaging time was achieved, which is the best long-term frequency stability of an all-fiber-based ultrastable laser observed to date.