We analyze ionic spectrum of lanthanum via intermediate state (Xe) $ 5d6d \; ^3F_2 $ in the energy region 89872-91783 cm ^–1, and the spectrum is obtained using five-laser resonance excitation in combination with a method of sequential ionization by a pulsed electric field and a constant electric field, and has been recalibrate in this work. Both of one strong and one weak autoionization Rydberg series converging to the La ²⁺state are determined. Meanwhile, the two autoionization Rydberg series are assigned by relativistic multichannel theory (RMCT) within the framework of multi-channel quantum defect theory (MQDT). More specifically, the strong autoionization Rydberg series is assigned to $ 5dnp\left(\dfrac{5}{2},\dfrac{1}{2}\right)_3 $ and/or $ 5dnp\left(\dfrac{5}{2},\dfrac{1}{2}\right)_2 $ , and the weak autoionization Rydberg series is assigned to $ 5dnf\left(\dfrac{5}{2},\dfrac{5}{2}\right)_3 $ and/or $ 5dnf\left(\dfrac{5}{2},\dfrac{5}{2}\right)_2 $ . We focus on the behavior of quantum defect with excitation energy for high $ n $ Rydberg states, which are sensitive to the existence of a external field. We find the breakdown of quantum defect regular behavior for a specific Rydberg series and autoionization Rydberg series of La ⁺as the effective quantum number $ n^\star>67 $ . Due to that our calculations, which are obtained by relativistic multichannel theory and included configuration interactions, are in basically agreement with that for experimental low $ n $ ( $ n^\star<67 $ ) Rydberg states as well as small stray electric fields, we suggest that plasma formed by photoionization of La atoms in the second excitation step may be responsible for the breakdown of quantum defect regular behavior.