  We proposed a new method to measure the energy spectrum of a superconducting
flux qubit. Different from the conventional frequency spectroscopy, a short
triangle pulse is used to drive the qubit through the anticrossing and
generates Landau-Zener-St\"uckelberg interference patterns, from which the
information of the energy spectrum can be extracted. Without installing
microwave lines one can simplify the experimental setup and reduce the unwanted
effects of noise. Moreover, the method can be applied to other quantum systems,
opening the possibility of calibrating and manipulating qubits with linear
pulses.
