EXHIBIT 12.1

Certification by the Chief Executive Officer

Pursuant to
Section 302 of the Sarbanes-Oxley Act of 2002

I, Shawn (Xiaohua) Qu, certify that:

1. I have reviewed this annual report on Form 20-F of Canadian Solar Inc. (the Company), as amended by Amendment
No. 1 to Form 20-F;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact
or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all
material respects the financial condition, results of operations and cash flows of the Company as of, and for, the periods presented in this report;

4. The Companys other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and
15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the Company and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the
Company, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding
the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the Companys disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and
procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this
report any change in the Companys internal control over financial reporting that occurred during the period covered by this annual report that has materially affected, or is reasonably likely to materially affect, the Companys internal
control over financial reporting; and

5. The Companys other certifying officer(s) and I have disclosed, based on
our most recent evaluation of internal control over financial reporting, to the Companys auditors and the audit committee of Companys board of directors (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting
which are reasonably likely to adversely affect the Companys ability to record, process, summarize and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the Companys internal control over financial reporting.

Date: October 26, 2011



|    |  |                        |  |                        
|--- |  | ---------------------- |  | -----------------------
|By: |  | /s/ Shawn (Xiaohua) Qu
|    |  | Name:                  |  | Shawn (Xiaohua) Qu     
|    |  | Title:                 |  | Chief Executive Officer