Aiming at the problem of the low utilization rate of equipment in the scheduling scheme due to ignoring the processing route constraints in the processing of multi-variety and small-batch complex products,a mathematical model of a multi-process flexible job shop was established with the goal of optimal completion time.Since the complexity of the model increased sharply after adding mul-tiple constraints,an improved atomic orbital search algorithm was proposed to solve the model effectively.In the improved algorithm,a three-layer coding method was used for encoding and decoding;the global machining route was generated uniformly when the algorithm initialized the candidate solutions;self-crossover was integrated into the search process to enhance local search;variable neighborhood mutation was introduced to avoid falling into local optimum;variable process number elite retention strategy was designed in the iterative process to expand the search space.Finally,the model and algorithm were solved and analyzed by an example of a diesel locomotive pro-duction workshop,which verifies the effectiveness of the model and the superiority and applicability of the algorithm.