Beijing Airport (PEK) to Jinghai by bus?

In the airport, you can take the express train to Sanyuanqiao Station, where you can transfer to subway line 10 to Shiliuzhuang Station. Take the Exit C and walk northwards about 0.7 miles, you will reach the Zhaogongkou Long-distance Bus Station. There are five buses available at 09:00, 11:00, 14:00, 16:00 and 18:00. The bus fare is CNY51.

I have another suggestion:
take the direct bus from PEK to Tianjin Tianhuan Bus Station. Then walk to the Infectious Disease Hospital bus stop to take Julong Express Bus or bus Jinglian Express Line to Jinghai directly.