Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 11, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Document Transition Report	false
Entity File Number	001-38697
Entity Registrant Name	PhaseBio Pharmaceuticals, Inc.
Entity Central Index Key	0001169245
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	03-0375697
Entity Address, Address Line One	1 Great Valley Parkway
Entity Address, Address Line Two	Suite 30
Entity Address, City or Town	Malvern
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	19355
City Area Code	610
Local Phone Number	981-6500
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	PHAS
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 120.5
Entity Common Stock, Shares Outstanding		29,443,225
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement, to be filed pursuant to Regulation 14A under the Securities Exchange Act of 1934, for its 2021 Annual Meeting of Stockholders are incorporated by reference in Part III of this Form 10-K.
Entity Filer Category	Non-accelerated Filer