需要下载mysql的源码，然后复制include文件夹到mamp目录，然后这样
gem install mysql2 -- --with-mysql-config=/Applications/MAMP/Library/bin/mysql_config --with-mysql-include=/Applications/MAMP/Library/include --with-mysql-lib=/Applications/MAMP/Library/lib
原文：http://www.cnblogs.com/ziyouchutuwenwu/p/4330837.html