If you don't have the invitation letter, you can't apply for a business visa (F visa), you can check the list of requirements for F visa application:
(I) Original Visa Notice Sheet from an authorized governmental apparatus /organization in China prescribing the visa validity, number of entries and stay period;

(II) Fully completed Visa Application Form in Chinese or English, attached with one of the applicant's recent color photograph (30mm X 40mm)

(III) Original passport with validity more than six months and minimum 2 blank visa pages;

(IV) Photocopy of the first 2 pages (personal information) of the passport and the recent Chinese visa page if there is any;

(V) Invitation letter from the company/organization/institution you want to visit in China;

If this is your first visit to China, it is recommended that following documents are also prepared in case of necessity:

(VI) Your income certificate/up-to-date Personal Bank Statement

(VII) Your company's Registration Certificate (photocopy)

(VIII) A recommendation letter from your company/organization/ institution

(IX) A recommendation letter from Chamber of Commerce & Industry

(X) Your Tax Payment Receipt for last year (photocopy)

(XI) Your Character Certificate/Police Clearance Certificate