字段有岐义，你得加它的别名前缀

select  a.xh,a.xm,a.TITLE,b.BJMC,b.XYMC,b.NJ 

from ypc_db.gxxs_tsgjc a 

LEFT JOIN 

(select 

XH,BJMC,XYMC,BJDM,NJ,LXDH 

from ypc_db.xsxxb 

union all 

select XH,BJMC,XYMC,BJDM,NJ,LXDH 

from ypc_db.xsxxb_other)

 b on a.XH=b.xh 

 where   1=1 

 ${if(len(XH) == 0,"","and a.XH = '" + XH + "'")}

 ${if(len(XM) == 0,"","and a.XM = '" + XM + "'")} 

 AND b.XH is  not NULL  

 and MONTH(time)=MONTH(SYSDATE())

 and  b.XYMC='${XYMC}'

条件中用XH字段帅选的时候，A,B两个表都存在型号字段，所以他不知道要用哪个表中的XH字段，在XH前注明表名就可以了。 ${if(len(XH) == 0,"","and a.XH = '" + XH + "'")}