  Quantum simulators with hundreds of qubits and engineerable Hamiltonians have
the potential to explore quantum many-body models that are intractable for
classical computers. However, learning the simulated Hamiltonian, a
prerequisite for any applications of a quantum simulator, remains an
outstanding challenge due to the fast increasing time cost with the qubit
number and the lack of high-fidelity universal gate operations in the noisy
intermediate-scale quantum era. Here we demonstrate the Hamiltonian learning of
a two-dimensional ion trap quantum simulator with 300 qubits. We employ global
manipulations and single-qubit-resolved state detection to efficiently learn
the all-to-all-coupled Ising model Hamiltonian, with the required quantum
resources scaling at most linearly with the qubit number. Our work paves the
way for wide applications of large-scale ion trap quantum simulators.
