In the application of laser heat treatment, when a laser beam is nearthe boundaries of a workpiece, it is important to avoid the melting dissolutionof the boundaries and obtain the satisfactory heat treatment. The experimentalresults indicates that designing the diaphragm in terms of boundaries′ shape ofa treated workpiece and then putting the diaphragm in the proper place, may avoidinjuring of boundaries effectively and obtain the treatment result with highquality. In order to simplify the research of heating action, treated material isusually regarded as the continuous medium whose thermal physical parameter isconstant. Moreover, when the boundaries of the treated workpiece are composed of the boundaries of intersectant planes, the appropriate image heat source can beused to deal with the problem of boundary. Therefore, instantaneous temperaturefield near the boundaries can be obtained satisfactorily by a semi-analyticnumeration of temperature field of laser heat treatment. To sum up, a simplemethod for boundary protection is proposed and proved by a semi-analyticnumeration of field of laser heat treatment in this paper. Results indicates thatchoosing the proper protective width of the boundaries properly, not only canprotect the boundaries effectively, but also can obtain the hardening band witheven depth near the boundaries.